Six-time champion Roger Federer remains undefeated at the ATP World Tour Finals following a tough three-set win over Marin Cilic.

Already assured of a record 14th semi-final appearance at the season-ending showpiece, Federer clinched top spot in his four-man group with a 6-7 (7-5) 6-4 6-1 comeback victory on Thursday.

Federer was frustrated not to have converted any of his five break-point chances until deep in the second set amid a run of five straight games to take command of the match.

He raced through the deciding set with two more service breaks to repeat his crushing victory over Cilic on London’s nearby grass courts in this year’s Wimbledon final.

“I was also a break point down early in the second set so I’m happy I found a way out of it,” Federer said.

“I played really well so I’m happy to be playing this level right now.”

The 19-time grand slam champion admitted it was a strange feeling knowing he wouldn’t be going home even if he had lost, but said the unusual scenario also freed him up to play his best tennis when he needed it.

“I tried to relax a little bit,” he said after completing the round-robin play unbeaten for the 10th time in his unparalleled career.

“But of course I want to do well. I want to try to keep the momentum going and play the right way and, to do that, a win’s always very helpful.”

Federer will face the winner of Friday’s clash between Dominic Thiem and David Goffin for a place in Sunday night’s championship decider.

“Both great guys, good friends of mine on the tour. I’ll watch that one tomorrow, try to relax now and then one more weekend, one more push, before the vacation and then the preparation for next season,” he said.

The winner of Thursday night’s match between American Jack Sock and German world No.3 Alexander Zverev will play Grigor Dimitrov in Saturday’s other semi-final at London’s O2 Arena.