LONDON (AP) — Roger Federer lost his opening match at the ATP Finals for the first time since 2013 as he went down 7-6 (4), 6-3 to Kei Nishikori on Sunday.
An error-strewn display from the 20-time Grand Slam champion allowed Nishikori to take the first set via a tiebreaker despite the Japanese player struggling for consistency himself.
Federer earned the first break points in the opening game of the second set, and took his chance, but Nishikori hit straight back in the following game.
Nishikori was far more composed than Federer from that moment on and broke for a 4-2 lead, before closing out the match for a first win over Federer since 2014.
Six-time champion Federer will now likely need to win his remaining two round-robin matches to keep alive his chances of claiming the 100th title of his career.
Earlier, Kevin Anderson made a confident start to his ATP Finals debut with a 6-3, 7-6 (10) victory over Dominic Thiem.
Having qualified for the season-ending tournament for the first time, the 32-year-old Anderson produced a dominant serving display to take the first set at the O2 Arena, before saving two set points to come through a tense tiebreaker in the second.
"I think it was important, you know, going out there and getting off to a good start," Anderson said. "I definitely felt a little bit nervous."
The fourth-seeded Anderson, who was runner-up at Wimbledon this year, was taken to deuce in his opening service game, but dropped only one more point on serve in the first set — and wasn't punished for taking just one of seven break point opportunities on Thiem's delivery.
Anderson's groundstrokes were almost as impressive as his serve, particularly in the first set, and drew a series of errors from Thiem in the fourth game, which resulted in the crucial break.
Thiem, who beat Anderson in straight sets at the U.S. Open this year, was struggling for consistency as he made just 48 percent of his first serves with 12 unforced errors, compared to just five winners.
"I didn't have a good start," Thiem said. "It was not working out at all."
The 25-year-old Austrian rallied in a far more competitive second set but couldn't force a break point despite taking Anderson to deuce on two occasions.
Thiem was showing glimpses of the form that took him to the French Open final earlier this year, but his unforced errors continued to bail out Anderson and the set went to a tiebreaker.
A rare triumph for Thiem in a lengthy rally gave him the first mini-break and a 2-1 lead, but Anderson quickly hit back before the pair exchanged mini-breaks once more to leave the score at 5-5.
Both players raised their level as 10 consecutive points, which included three match points for Anderson and two set points for Thiem, went with serve.
Anderson, who saved a match point before knocking Federer out in the Wimbledon quarterfinals in July, once more displayed his ability to thrive under pressure.
Forced wide by Thiem, Anderson slapped a forehand winner up the line to bring up a first match point on his own serve at 11-10.
"The more I'm in those positions," Anderson said, "I definitely feel more and more comfortable."
He didn't waste it, hitting a 13th ace of the match to ensure that the sixth-seeded Thiem has now lost his opening round-robin match on all three of his appearances at the tournament.
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