Former British Davis Cup player on BBC Radio 5 Live Sports Extra
The sign that Federer's tactics were spot on was that Djokovic wasn't able to get comfortable. It was like a boxer where the other guy wasn't there to hit. That's how Djokovic will feel right now.
AFPCopyright: AFP
Post update
Djokovic 4-6 3-6 Federer
Roger Federer on what was different from Wimbledon: "I won match point I guess! It was so close at Wimbledon. It was a privilege to play that match, so many ups and downs. I couldn't be more happy right now.
"You always play better as the tournament goes on. Thiem has been supreme in our group, made life difficult for both of us, and made it so that it was winner goes through. I think I served great, had great anticipation, clear game-plan and it worked great tonight - hopefully not for the last time against Novak.
"I'm happy to stick around. I don't think we know who I play so I'll watch some tomorrow and get ready then hopefully play some good tennis at the weekend."
Post update
Djokovic 4-6 3-6 Federer
Russell Fuller
BBC tennis correspondent on Radio 5 Live Sports Extra
It was an immaculate performance. Federer said it himself he played incredible and who are we to argue. It was the perfect performance at the perfect time.
Post update
Djokovic 4-6 3-6 Federer
Roger Federer, speaking on court: "Great atmosphere, great opponent. It was definitely incredibly special. I enjoyed it from the beginning. I played incredible and I knew I had to because that's what Novak does. It was definitely magical. You guys made it super special, I can't thank you enough."
Post update
Djokovic 4-6 3-6 Federer
That was about as a good a serving performance as you could hope to see from Roger Federer.
Twelve aces, 23 winners, 81% of first serve points won and 69% of second serve points won.
PA MediaCopyright: PA Media
Post update
Djokovic 4-6 3-6 Federer
That defeat means that Rafael Nadal will end the year as world number one.
Game, set and match - Federer
Djokovic 4-6 3-6 Federer
Into the net goes Novak Djokovic!
Roger Federer leaps into the air in delight - he's into the semi-finals once again.
Post update
*Djokovic 4-6 3-5 Federer
Too much on these forehand swipes from Djokvoic, twice dragging the ball out of bounds, and then a horror of a miss brings up three match points for Federer...
Getty ImagesCopyright: Getty Images
Djokovic serving to stay in match
*Djokovic 4-6 3-5 Federer
Here we go...
Post update
*Djokovic 4-6 3-5 Federer
A huge roar as Federer leaps into the air and pops home a volley to start his latest service game.
Down comes an ace and then, thanks to a Federer challenge, the game is closed out with another.
Post update
Djokovic 4-6 3-4 Federer*
Russell Fuller
BBC tennis correspondent on Radio 5 Live Sports Extra
It's been a special night - one of the best atmospheres I can ever remember at this event.
ReutersCopyright: Reuters
Post update
Djokovic 4-6 3-4 Federer*
Some very good forehands from Djokovic are followed by some not so great stuff, but a delicious drop shot from down off his ankles helps him spin to a 40-15 lead.
Into the net goes Federer from out wide, and that's a fine hold.
Federer holds
*Djokovic 4-6 2-4 Federer
There's winners all over the show as Federer gets himself to a 40-0 lead but Djokovic won't go down that easily, sending a forehand of his own flying past the Swiss for 40-30.
Big hitting from Federer, shot after shot coming Djokovic's way, and he slips out of the rally as Federer holds.
PA MediaCopyright: PA Media
Federer breaks
Djokovic 4-6 2-3 Federer*
Well then!
A baseline exchange and Djokovic can only whip a forehand long.
Federer breaks.
Post update
*Djokovic 4-6 2-2 Federer
A good one-two from Djokovic, and a sprawling Federer can't find a response.
15-40.
EPACopyright: EPA
Post update
*Djokovic 4-6 2-2 Federer
Some messy errors from Djokovic, first sending the ball out off the top of the racquet before overworking an attempted forehand.
It's 0-30, but it's quickly 0-40 thanks to the ball nestling in the foot of the net.
Federer has three break points...
Federer holds
*Djokovic 4-6 2-2 Federer
Djokovic gets a look at Federer's second serve and steps in to the court to take it early... before shanking his return into the net.
Bosh. A first serve from Federer that Djoko can't return.
Live Reporting
Amy Lofthouse
All times stated are UK
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Latest PostGoodbye!
Right, that's us done for a bit
The report of this evening's match is over here and you can catch up with what went down earlier today here.
Join us from 14:00 GMT tomorrow for coverage of the fifth day in London.
In a bit!
Post update
Post update
Roger Federer is likely to face Stefanos Tsitsipas in the semi-finals, with the Greek expected to top the group.
All three of the remaining players in that group can qualify for the semi-finals, as this handy graphic explains...
Post update
Miles Maclagan
Former British Davis Cup player on BBC Radio 5 Live Sports Extra
The sign that Federer's tactics were spot on was that Djokovic wasn't able to get comfortable. It was like a boxer where the other guy wasn't there to hit. That's how Djokovic will feel right now.
Post update
Djokovic 4-6 3-6 Federer
Roger Federer on what was different from Wimbledon: "I won match point I guess! It was so close at Wimbledon. It was a privilege to play that match, so many ups and downs. I couldn't be more happy right now.
"You always play better as the tournament goes on. Thiem has been supreme in our group, made life difficult for both of us, and made it so that it was winner goes through. I think I served great, had great anticipation, clear game-plan and it worked great tonight - hopefully not for the last time against Novak.
"I'm happy to stick around. I don't think we know who I play so I'll watch some tomorrow and get ready then hopefully play some good tennis at the weekend."
Post update
Djokovic 4-6 3-6 Federer
Russell Fuller
BBC tennis correspondent on Radio 5 Live Sports Extra
It was an immaculate performance. Federer said it himself he played incredible and who are we to argue. It was the perfect performance at the perfect time.
Post update
Djokovic 4-6 3-6 Federer
Roger Federer, speaking on court: "Great atmosphere, great opponent. It was definitely incredibly special. I enjoyed it from the beginning. I played incredible and I knew I had to because that's what Novak does. It was definitely magical. You guys made it super special, I can't thank you enough."
Post update
Djokovic 4-6 3-6 Federer
That was about as a good a serving performance as you could hope to see from Roger Federer.
Twelve aces, 23 winners, 81% of first serve points won and 69% of second serve points won.
Post update
Djokovic 4-6 3-6 Federer
That defeat means that Rafael Nadal will end the year as world number one.
Game, set and match - Federer
Djokovic 4-6 3-6 Federer
Into the net goes Novak Djokovic!
Roger Federer leaps into the air in delight - he's into the semi-finals once again.
Post update
*Djokovic 4-6 3-5 Federer
Too much on these forehand swipes from Djokvoic, twice dragging the ball out of bounds, and then a horror of a miss brings up three match points for Federer...
Djokovic serving to stay in match
*Djokovic 4-6 3-5 Federer
Here we go...
Post update
*Djokovic 4-6 3-5 Federer
A huge roar as Federer leaps into the air and pops home a volley to start his latest service game.
Down comes an ace and then, thanks to a Federer challenge, the game is closed out with another.
Post update
Djokovic 4-6 3-4 Federer*
Russell Fuller
BBC tennis correspondent on Radio 5 Live Sports Extra
It's been a special night - one of the best atmospheres I can ever remember at this event.
Post update
Djokovic 4-6 3-4 Federer*
Some very good forehands from Djokovic are followed by some not so great stuff, but a delicious drop shot from down off his ankles helps him spin to a 40-15 lead.
Into the net goes Federer from out wide, and that's a fine hold.
Federer holds
*Djokovic 4-6 2-4 Federer
There's winners all over the show as Federer gets himself to a 40-0 lead but Djokovic won't go down that easily, sending a forehand of his own flying past the Swiss for 40-30.
Big hitting from Federer, shot after shot coming Djokovic's way, and he slips out of the rally as Federer holds.
Federer breaks
Djokovic 4-6 2-3 Federer*
Well then!
A baseline exchange and Djokovic can only whip a forehand long.
Federer breaks.
Post update
*Djokovic 4-6 2-2 Federer
A good one-two from Djokovic, and a sprawling Federer can't find a response.
15-40.
Post update
*Djokovic 4-6 2-2 Federer
Some messy errors from Djokovic, first sending the ball out off the top of the racquet before overworking an attempted forehand.
It's 0-30, but it's quickly 0-40 thanks to the ball nestling in the foot of the net.
Federer has three break points...
Federer holds
*Djokovic 4-6 2-2 Federer
Djokovic gets a look at Federer's second serve and steps in to the court to take it early... before shanking his return into the net.
Bosh. A first serve from Federer that Djoko can't return.