Roger Federer ended Novak Djokovic’s 15-game winning streak at the ATP World Tour Finals with a 7-5 6-2 victory to advance to the semi-finals with one match remaining.
Djokovic, winner of the past three season-ending titles in London, had won his previous 38 indoor matches, and has been unbeaten on the indoor courts since 2012.
However, the Serb had no answer to the world number two who won comfortably in the end in two sets to become the first man to reach the last four.
Federer won 75 per cent (27 of 36) of his first-serve points, and converted four of eight break-point opportunities.
Djokovic, 28, had not been beaten anywhere since he lost to the Swiss in Cincinnati three months ago, and the world number one’s last defeat at the O2 came in 2011.
After the game, the 34-year old Swiss said: “Winning the first set clearly in a match like this gives you the upper hand, it was important to strike again as quick as possible in the second set, and I did.
“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.
“The tactic was to mix it up, I have to against Djokovic.
“I hit it well out wide today. I knew it was an important serve to get Novak off the court.”
The world number one who gave credit to his Swiss counterpart, conceded he made lots of mistake to hand Federer the victory.
Djokovic said: “You have those days when you are not feeling your best, not even close to the best.
“Credit to Roger for mixing up the pace, giving me always a different ball. He used the slice and spin very wisely. He served very efficiently.
“I made a lot of, lot of unforced errors. I just handed him the win, especially in the second set.
“Sometimes these kind of matches, these kind of days happen.
“The good thing is that it’s a round-robin system, so I still have an opportunity to reach the semis.
“Winning the first set clearly in a match like this gives you the upper hand, it was important to strike again as quick as possible in the second set, and I did.”
The rivals have met more times (43) than any other two players in the Open Era with the exception of Djokovic and Rafael Nadal (45).
Federer will play Japan’s Kei Nishikori, who beat Berdych 7-5 3-6 6-3 in the afternoon session.
Djokovic will now face Berdych on Thursday for a spot in the semis.