Rory McIlroy told BBC Sport he made "too many mistakes" in his topsy-turvy round of 73.
"I made six bogeys out there," he said. "I made enough birdies, I made five birdies, but I just made too many mistakes so I'm just going to go and address a couple of those little issues.
"It would have been great to shoot under par, but it's not as if anyone is getting away out there.
"Even though three under is only leading I thought the course was there. It's soft, you could take advantage of those conditions and I just didn't do that.
"I could be starting Friday a couple of shots better but I just have to get off to a good start and try to get myself back into red figures."
'I knew what I needed to do'
Poulter -4 (F)
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As things stand Ian Poulter is just two strokes back off the lead and has given himself a pretty solid platform to eclipse his best ever finish here, tied for sixth in 2015. So what did he make of it all?
"I played great today," he said.
"It is nice to get off to a decent start. There was one
bogey, one mistake on nine but tee to green I kept giving myself plenty of chances, it was very
solid.
"I’ve played many rounds here now. Coming up I had a relaxed
feeling and I feel comfortable on the golf course and I knew going out there what I needed to do."
'Tee to green I was better last year'
Rahm -3 (F)
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Jon Rahm, an early clubhouse leader, speaking earlier:
"It’s very different to what it is last year. I was hitting
it better off the tee and my iron game was sharper. Tee to green I was better last year but my putting has been
better and been more consistent this year.
"You may think I was calm but I’m working on it. I’ve been working
on what my body is saying and portraying. I try to not get too down on anything or too concerned and that
has helped keep my rounds less up and down and a lot more even."
A shock slump...
Casey +9 (18)
Golf writer Brian Wacker sums up the surprise nature of the Paul Casey woe...
Not exactly sure why Jason Day was picking his ball out of the hole either. That can't have helped his hip/lower back issue...
Post update
Day -2 (F)
Shaun Micheel
2003 US PGA Championship winner on BBC Radio 5 live
Jason Day is such a strong looking kid but he is always injured. What is he doing that is aggravating his injury? When you get an injury you are meant to rest.
I don't know what he is doing off the course but he has got this kind of reputation now when things aren't going well of having tweaked his back. I don't know what he is doing off the course and it is easy to throw stones.
Legitimate or not he has to try and find a way to play through it, not talk about it or just take some time off.
'Everyone was going crazy'
DeChambeau -6 (F)
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Bryson DeChambeau is more than a little upbeat in the clubhouse. Here's what he's had to say about his six under par opening round - which makes him the joint leader.
"The shot on 18 came out of the rough beautifully. I needed the bounce, it rolled up and I thought
I’d made it. Everyone was going crazy.
"I just for some reason got some good numbers going my way (on the back nine) and I
hit my shots beautifully. We changed every shaft on the clubs leading into this, it's pretty cool to understand the
dynamics around this and there is something special to come."
Post update
Thought the big-name Americans were supposed to be out of form?
Leaderboard
-6 B DeChambeau (US), B Koepka (US)
-5 P Mickelson (US)
-4 I Poulter (Eng), D Johnson (US)
-3 J Harding (SA), A Scott (Aus), J Rahm (Spa), K Kisner (US), K Aphibarnrat (Tha)
-2 L Bjerregaard (Den), C Conners (Can), J.B. Holmes (US), G Woodland (US), T Woods (US), R Fowler (US), C Smith (Aus), P Kizzire (US), J Day (Aus), F Molinari (Ita)
Selected others:
-1 T Finau (US), T Fleetwood (Eng),L Oosthuizen (SA), T Olesen, (Den) B Langer, (Ger)
E H Li (Chn), B Watson (US)
+1 R McIlroy (NI), S Garcia (Spa), P Reed (US), S Lyle (Sco), T Hatton (Eng)
+2 E Pepperell (Eng), H Stenson (Swe)
+3 D Willett (Eng) M Wallace (Eng), J Rose (Eng), J Spieth (US), M Wallace (Eng)
+8 I Woosnam (Wal)
+9 P Casey (Eng)
Post update
So here is how the land lies after day one at Augusta...
Post update
Spieth +3, Casey +9, Koepka -6 (18)
Not such a good day for Paul Casey. And that's putting into very mildly.
The Englishman carded five bogeys and two double bogeys - including that one at the last - to finish only ahead of former champion Angel Cabrera.
Someone in the BBC Sport office stuck a fiver each way on him...
Post update
Spieth +3, Casey +9, Koepka -6 (18)
Not quite the perfect finish for Koepka. His birdie putt on the last, swings left to right and misses the hole.
Never mind. The muscle man has barged his way to the top of the leaderboard, alongside Bryson DeChambeau, after a bogey-free 66.
Post update
Spieth +3, Casey +7, Koepka -6 (17)
Casey has the putter out. He's needs a Langer-esque effort here. Face palm... he's only made the fringe.
Koepka, pin high and eight feet to the left of the stick, waits patiently as his playing partners try to end their messy rounds.
Spieth's effort is decent, just finishing short.
Woods-like Koepka
Koepka -6 (17)
Iain Carter
BBC Sport golf correspondent at Augusta National
Koepka comes alive in the majors. It's extraordinary. He's done nothing all year.
Everyone had a go at him for losing 24lbs, people saying he'd lost his golf game. Now he looks fantastic.
He's won three of his last six majors. It's Tiger-Woods like strike rate.
Post update
Spieth +3, Casey +7, Koepka -6 (17)
Jordan Spieth hasn't had much to smile about tonight, nor this year, but he's got a chance to finish with a birdie on the last.
Playing partner Paul Casey, however, hasn't. The Englishman was talking a good game in the build-up - and in good form - but he's had a bad day at the office today. Now he's through the back for three...
Birdie for Mickelson
Mickelson -5, Rose +3, Thomas E (18)
Nailed it! Phil Mickelson knew where this was going as soon as it left the face... birdie town. A wonderful finish from the American great - five birdies in the final seven holes - leaves him one off the lead.
Phil Mick is 48 years and nine months... no-one, not even Jack Nicklaus, has been that old when they've won the Masters...
Post update
Mickelson -4, Rose +3, Thomas E (17)
Another birdie to finish will help Justin rose sleep a little easier tonight. The Englishman gives it a good shove from the brow of the slope, but it juts slows down at the last second or two. A par will have to do.
The stage is set for Leftie...
Post update
Koepka -6 (17)
Meanwhile, back down the hill... co-leader Brooks Koepka almost knocks the paint off his ball from the tee. Well over 300 yards, middle of the fairway....
Post update
Mickelson -4, Rose +3, Thomas E (17)
Mickelson likes his approach... and so he should. It pitches a few feet short of the pin before spinning back and checking. Birdie chance...
Live Reporting
Jonathan Jurejko and Luke Reddy
All times stated are UK
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Latest PostGoodbye!
Right then, that's day one done and dusted. Been fun hasn't it?
Join us again tomorrow to see if the big names high on the bill continue to perform.
In the meantime, don't forget you can watch highlights tomorrow (full deets here) and catch up with all the main talking points by reading Pete Scrivener's report from Augusta.
Manana!
McIlroy made 'too many mistakes'
Rory McIlroy told BBC Sport he made "too many mistakes" in his topsy-turvy round of 73.
"I made six bogeys out there," he said. "I made enough birdies, I made five birdies, but I just made too many mistakes so I'm just going to go and address a couple of those little issues.
"It would have been great to shoot under par, but it's not as if anyone is getting away out there.
"Even though three under is only leading I thought the course was there. It's soft, you could take advantage of those conditions and I just didn't do that.
"I could be starting Friday a couple of shots better but I just have to get off to a good start and try to get myself back into red figures."
'I knew what I needed to do'
Poulter -4 (F)
As things stand Ian Poulter is just two strokes back off the lead and has given himself a pretty solid platform to eclipse his best ever finish here, tied for sixth in 2015. So what did he make of it all?
"I played great today," he said.
"It is nice to get off to a decent start. There was one bogey, one mistake on nine but tee to green I kept giving myself plenty of chances, it was very solid.
"I’ve played many rounds here now. Coming up I had a relaxed feeling and I feel comfortable on the golf course and I knew going out there what I needed to do."
'Tee to green I was better last year'
Rahm -3 (F)
Jon Rahm, an early clubhouse leader, speaking earlier:
"It’s very different to what it is last year. I was hitting it better off the tee and my iron game was sharper. Tee to green I was better last year but my putting has been better and been more consistent this year.
"You may think I was calm but I’m working on it. I’ve been working on what my body is saying and portraying. I try to not get too down on anything or too concerned and that has helped keep my rounds less up and down and a lot more even."
A shock slump...
Casey +9 (18)
Golf writer Brian Wacker sums up the surprise nature of the Paul Casey woe...
Post update
Not exactly sure why Jason Day was picking his ball out of the hole either. That can't have helped his hip/lower back issue...
Post update
Day -2 (F)
Shaun Micheel
2003 US PGA Championship winner on BBC Radio 5 live
Jason Day is such a strong looking kid but he is always injured. What is he doing that is aggravating his injury? When you get an injury you are meant to rest.
I don't know what he is doing off the course but he has got this kind of reputation now when things aren't going well of having tweaked his back. I don't know what he is doing off the course and it is easy to throw stones.
Legitimate or not he has to try and find a way to play through it, not talk about it or just take some time off.
'Everyone was going crazy'
DeChambeau -6 (F)
Bryson DeChambeau is more than a little upbeat in the clubhouse. Here's what he's had to say about his six under par opening round - which makes him the joint leader.
"The shot on 18 came out of the rough beautifully. I needed the bounce, it rolled up and I thought I’d made it. Everyone was going crazy.
"I just for some reason got some good numbers going my way (on the back nine) and I hit my shots beautifully. We changed every shaft on the clubs leading into this, it's pretty cool to understand the dynamics around this and there is something special to come."
Post update
Thought the big-name Americans were supposed to be out of form?
Leaderboard
-6 B DeChambeau (US), B Koepka (US)
-5 P Mickelson (US)
-4 I Poulter (Eng), D Johnson (US)
-3 J Harding (SA), A Scott (Aus), J Rahm (Spa), K Kisner (US), K Aphibarnrat (Tha)
-2 L Bjerregaard (Den), C Conners (Can), J.B. Holmes (US), G Woodland (US), T Woods (US), R Fowler (US), C Smith (Aus), P Kizzire (US), J Day (Aus), F Molinari (Ita)
Selected others:
-1 T Finau (US), T Fleetwood (Eng),L Oosthuizen (SA), T Olesen, (Den) B Langer, (Ger)
E H Li (Chn), B Watson (US)
+1 R McIlroy (NI), S Garcia (Spa), P Reed (US), S Lyle (Sco), T Hatton (Eng)
+2 E Pepperell (Eng), H Stenson (Swe)
+3 D Willett (Eng) M Wallace (Eng), J Rose (Eng), J Spieth (US), M Wallace (Eng)
+8 I Woosnam (Wal)
+9 P Casey (Eng)
Post update
So here is how the land lies after day one at Augusta...
Post update
Spieth +3, Casey +9, Koepka -6 (18)
Not such a good day for Paul Casey. And that's putting into very mildly.
The Englishman carded five bogeys and two double bogeys - including that one at the last - to finish only ahead of former champion Angel Cabrera.
Someone in the BBC Sport office stuck a fiver each way on him...
Post update
Spieth +3, Casey +9, Koepka -6 (18)
Not quite the perfect finish for Koepka. His birdie putt on the last, swings left to right and misses the hole.
Never mind. The muscle man has barged his way to the top of the leaderboard, alongside Bryson DeChambeau, after a bogey-free 66.
Post update
Spieth +3, Casey +7, Koepka -6 (17)
Casey has the putter out. He's needs a Langer-esque effort here. Face palm... he's only made the fringe.
Koepka, pin high and eight feet to the left of the stick, waits patiently as his playing partners try to end their messy rounds.
Spieth's effort is decent, just finishing short.
Woods-like Koepka
Koepka -6 (17)
Iain Carter
BBC Sport golf correspondent at Augusta National
Koepka comes alive in the majors. It's extraordinary. He's done nothing all year.
Everyone had a go at him for losing 24lbs, people saying he'd lost his golf game. Now he looks fantastic.
He's won three of his last six majors. It's Tiger-Woods like strike rate.
Post update
Spieth +3, Casey +7, Koepka -6 (17)
Jordan Spieth hasn't had much to smile about tonight, nor this year, but he's got a chance to finish with a birdie on the last.
Playing partner Paul Casey, however, hasn't. The Englishman was talking a good game in the build-up - and in good form - but he's had a bad day at the office today. Now he's through the back for three...
Birdie for Mickelson
Mickelson -5, Rose +3, Thomas E (18)
Nailed it! Phil Mickelson knew where this was going as soon as it left the face... birdie town. A wonderful finish from the American great - five birdies in the final seven holes - leaves him one off the lead.
Phil Mick is 48 years and nine months... no-one, not even Jack Nicklaus, has been that old when they've won the Masters...
Post update
Mickelson -4, Rose +3, Thomas E (17)
Another birdie to finish will help Justin rose sleep a little easier tonight. The Englishman gives it a good shove from the brow of the slope, but it juts slows down at the last second or two. A par will have to do.
The stage is set for Leftie...
Post update
Koepka -6 (17)
Meanwhile, back down the hill... co-leader Brooks Koepka almost knocks the paint off his ball from the tee. Well over 300 yards, middle of the fairway....
Post update
Mickelson -4, Rose +3, Thomas E (17)
Mickelson likes his approach... and so he should. It pitches a few feet short of the pin before spinning back and checking. Birdie chance...