Roger Federer has withstood a stunning Kei Nishikori fightback to remain undefeated at the ATP World Tour Finals.

The 34-year-old Swiss maestro was on auto-pilot when he led 4-1 in the second set, having snatched an absorbing opener, but needed every shot in his formidable armoury to eventually claw out a 7-5 4-6 6-4 victory.

Japan’s Nishikori had begun to look the more likely winner after reeling off five consecutive games to take the six-times former champion into a deciding set, but his stirring efforts were in vain as Federer squeezed home.

Federer wins Stan Smith Group with a 100 per cent record – the ninth time in 14 consecutive appearances at the elite event he has won all three of his round-robin matches.

World number eight Nishikori bowed out having only beaten Tomas Berdych, while favourite and defending champion Novak Djokovic will take the runners-up spot if he beats Berdych later at the O2 Arena, or avoids a straight sets loss.

“It was extremely difficult. It was a good match, with more breaks than we are used to. Kei is a very good returner, especially on the second serve. It was a very physical match, but I enjoyed it,” Federer said on court.

“Kei was staying aggressive and going for his shots and that makes it hard. I had to hope that he would slip.”

Federer will not know his semi-final opponent until Ilie Nastase Group is concluded on Friday with the match between Britain’s Andy Murray and compatriot Stanislas Wawrinka.

Rafael Nadal has already won the other group.