After cancelling his training session a day earlier, Federer produced a much-improved performance to get his campaign for a seventh ATP Finals title back on track with a 6-2, 6-3 victory over Dominic Thiem on Tuesday.
The Swiss star suffered a shock opening-round defeat against Nishikori before seeing off Thiem in straight sets in round two.
He kept alive his hopes of advancing from his group with 6-2 6-3 win over the Austrian.
Yet 48 hours later, the 37-year-old Federer rebounded in style, to the delight of the crowd packed into The O2 arena. Sometimes I get asked, 'do you want to play day or night? "I hope I showed it". "That's okay. If I don't pass, I don't pass, and I deserve not to pass. Alexander Zverev fumes at major issue after Novak Djokovic loss Novak Djokovic reveals true thoughts of new ATP Cup - full format "Important for me is to play a good last match here in the round-robin, try to beat Kevin".
"I don't know what happened against Nishikori. I love playing in London, always have, always will".
However, the 20-time grand slam champion still needs to beat the in-form Kevin Anderson, who knocked him out at Wimbledon and who dropped just one game against Nishikori in Tuesday afternoon's match, if he is to clamber out of the group.
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He certainly looked back in the groove, making only 11 unforced errors, compared to the 34 against Nishikori, and surrendering just six points on serve. The Swiss superstar has an impressive history at the end of year event.
Federer turned the screws to break again immediately in the second set and never looked in any danger.
With this result, the semi-final scenarios in the Lleyton Hewitt group got a little more complicated as no player has sealed his berth in the last four, nor has anyone been eliminated, with Anderson still being the front-runner.
He produced a near-perfect display against Nishikori who he lost to two weeks ago in Paris.
"That's amongst the matches best I've played", said Anderson.
"I felt fresh, but I just didn't feel the ball this week", said Nishikori. "I could win and still not make it, so from that standpoint it's not in my power", Federer said.