Federer rallied to save three set points, but Nishikori delivered a well-placed serve out wide to ensure the comeback attempt ended there.

However, his concentration deserted him moments later as a double fault gave Federer a break he did little to earn. But the Swiss responded in kind and a wild forehand error leveled the score at 1-1.

"Unfortunately I couldn't keep the lead that I got early (in the second set),"Federer said. "That was the key of the match, that sort of 10-minute swing at the end of the first through to maybe 1-1 in the second." The seventh-seeded Nishikori began to relax and was rewarded in the sixth game as he brought up another break point and produced an inside-out forehand Federer had no chance of returning.

Despite having lost his last six meetings with Federer, including two on this court, Nishikori showed no nerves as he dropped just one point in his next two service games, with a forehand error from the Swiss great ending the match.

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 tournament for the first time, the 32-year-old Anderson produced a dominant serving display to take the first set, 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."