Roger Federer wins 7-5 6-2 in London
Federer into semi-finals with second win
Swiss leads head-to-head 22-21
Both won their opening group matches
That's the end of another enjoyable day at the 02. Hopefully the standard will be just as high on Wednesday when British number one Andy Murray takes on Rafael Nadal at around 14:00 GMT. It feels like an age since we last saw the two of them going at it. Can Nadal beat a big gun? The winner will reach the semi-final.
There will be live coverage of that match via live text, radio and tv so do join us. Until tomorrow...
Image source, Getty ImagesDjokovic 5-7 2-6 Federer
"I hit it well out wide today. I knew it was an important serve to get Novak off the court," Federer tells Sky Sports.
Image source, Getty ImagesDjokovic 5-7 2-6 Federer
Federer's victory means he has qualified for the semi-finals. Djokovic must beat Tomas Berdych, who himself still has a chance of qualifying, to guarantee his spot in the last four.
Djokovic 5-7 2-6 Federer
Djokovic had won 15 straight matches at the ATP World Tour Finals - but Federer has brought an end to that winning streak, too.
Currently at the 02, Stefan Edberg is being inducted into the Tennis Hall of Fame and has received a knuckle-breaking ring.
Djokovic 5-7 2-6 Federer
Federer's victory brings an end to Djokovic's 38-match winning run indoors. It also ends the world number one's 23-match win run since he lost to Federer in Cincinnati final on 23 August.
Matt McGladrigan:#Djokovic, external probably doubled his 2015 error count in that last 20 minutes. #FinalShowdown, external.
Joel Newnham: Who thinks these two will meet in the final?
Djokovic 5-7 2-6 Federer
"It's always a thrill to come out here, even more so against a great player like Novak. It's a special night for tennis, and a cool atmosphere," says Federer
Asked what tactics worked well, Federer tells Sky Sports: "I won't tell you because we might play again." After the laughter dies down, the Swiss expands: "The tactic was to mix it up. I have to against Djokovic."
Image source, AFP
Djokovic 5-7 2-6 Federer
How long can Djokovic hang on? That question is soon answered. The Serb goes long with a forehand. Second match point Federer.
Djokovic steps in on the next point... He eyes the winner, but that forehand looked wide. It's called out. Djokovic challenges. Tension, drama before the curtain is drawn.
And replays proves the ball bounced wide. A first defeat indoors for the world number one since 2012.
Djokovic saves match point
Magical. A beaut of a backhand return from Federer brings up his first match point.
Sometimes there are no words, sometimes you just have to watch and admire. The longest rally of the match begins... silence as the pair battle it out for 23 shots, 24 shots, 25 shots... Federer into the net. His wife, Mirka, winces.
Djokovic lives on. But Federer reels him in from advantage with a thumping forehand into the corner. Deuce.
Miles Maclagan
British tennis coach on Radio 5 live sports extra
Quote MessageNovak Djokovic's mental state is maybe not as tough as we are used to seeing. There have been six or seven shots in the last couple of games where he's fallen below his usual standards.
Can Djokovic come back from here? You wouldn't bet against it. The Serb usually has infinite reserves of energy, but he's just been a touch off colour this evening, winning just 29% of points on his first serve in the second set.
The backhand is his enemy, too, another botched shot allows Federer to level at 30-30. The crowd willing the Swiss on. Djokovic rattled.
Spectators sigh in unison as they watch a Federer backhand loop towards the ceiling. But a tired-looking forehand into the tape from Djokovic takes us to deuce.
Djokovic is making too many errors. The Serb looks skywards, but he needs to look at his groundstrokes for that is where the problem lies. The Serb's feet aren't moving in unison, a wild swing at a forehand, and Federer holds to love. He's within touching distance. Djokovic's 23-match winning run could soon come to an end.
Image source, AFPFederer breaks
Opening blow to Federer. Djokovic rolls his eyes, takes an deep intake of breath. The Serb is not amused.
Djokovic coming into the net... A serve and volley...Federer, feeling the pressure, nets. Djokovic embarks on an angry monologue despite winning the point to level at 15-15.
The red mist playing havoc with Djokvic's mind. He's lost his cool, and two points. Quickly. Two breakers. And Federer takes them with ease. A peachy backhand down the line. The Swiss with acres of the court to aim at.
There's no weakness in Federer's serve. His forehand could do with a talking to, though, as an iffy balloons wide for 40-30, giving Djokovic a sniff. But Federer simply marches on without fuss.
Image source, ReutersSophie Bonner: Tense but backing @rogerfederer, external for the win! #bbctennis, external amazing tennis tonight!!
For a moment, for a millisecond, Djokovic showed a hint of vulnerability at the start of this set, but other than a dreamy Federer winning return, the Serb has little trouble holding. His first serve percentage has dropped, but the second serve is solid enough.
The pair exchange blows from the back of the court on the final point... Federer looks to be gaining an advantage, but Djokovic nonchalantly places a backhand winner beyond the Swiss' reach to hold.