London: Even the best player in the world can have an off day, especially when he's playing someone many believe is the best of all time.
Roger Federer brought his top game to the O2 Arena on Tuesday, stopping Novak Djokovic's ATP Finals winning streak at 15 by beating the top-seeded Serb 7-5, 6-2.
"I wouldn't have picked it maybe before the tournament, just because of his really good record on the indoors, the year he's had," said Federer, who lost to Djokovic in the Wimbledon and U.S. Open finals. "But then again, all day I felt good about my chances."
The win put Federer in the semifinals of the season-ending tournament for the 13th time in 14 appearances.
Djokovic, who has already clinched the year-end No. 1 ranking, had not lost at the ATP Finals since 2011, winning the last three elite titles for the top eight players in the world.
But Federer was strong Tuesday, as he usually is on an indoor hard court.
The six-time champion broke Djokovic in the final game of the first set, and again early in the second. Although Djokovic broke right back, Federer regained the lead a short time later and won the final four games.
"Yeah, you have those days as well, when you are not feeling your best, not even close to the best," Djokovic said. "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 unforced errors. Just handed him the win."
On Sunday, Federer beat Tomas Berdych, giving him a 2-0 record this week. He will still play Kei Nishikori on Thursday in the final round-robin match.
Djokovic beat Nishikori in his first match, and will face Berdych on Thursday for a spot in the semifinals.
Djokovic has been the best player in tennis all year, winning three Grand Slam titles and losing the other in the final at the French Open. In beating Federer at Wimbledon and the U.S. Open, he won his ninth and 10th major titles.
He found it tough to get going against the 17-time Grand Slam champion on Tuesday, though.
"These days happen. These kind of things happen," Djokovic said. "I have to accept it and hopefully work on it tomorrow and get better next time."
In the early match, Nishikori's 7-5, 3-6, 6-3 victory over Berdych came down to one point.
With the score even at 3-3 in the third set, the sixth-seeded Berdych had his third break point of the match — after converting the previous two — but he put a backhand into the net after a long rally.
"That was a really big point. He had three, four shots that he went really aggressive," said Nishikori, who reached the semis last year in his tournament debut. "I mean, last shot, I kind of went for it, closed my eyes. Was a little bit lucky I made that shot."
Berdych attributed the loss of the match to the loss of that point.
"The rally starts pretty well for me, but I was not able to break him. Then I lost my serve," said Berdych, who has been defeated twice this week. "Basically, that was the difference today."
After holding serve in that game, Nishkori broke the Czech to take a 5-3 lead, and served out.
In the first set, Nishikori took the lead with a late break at love, and held serve. He broke again to start the second set and led 2-0, but Berdych won five straight games and eventually the set.
Nishikori is 1-1 after losing to Djokovic on Sunday, and he still has to face Federer.
"It's going to be a really important match for both of us," Nishikori said. "I have to win the next match to qualify (from) the group. So I (will) try to recover well today and tomorrow, and I will get good tactics ready."
