Roger Federer storms back to beat Marin Cilic and complete group stage clean sweep

Cilic impressed in a high-quality first set that went all the way to a tie-break. But Federer upped his level significantly from then on, steaming through the second and third sets to make it three wins from three in the group stage.

Federer's spot in the final four of the tournament was already sealed prior to Thursday's match thanks to wins over Alexander Zverev and Jack Sock. And when finding himself a set behind and visibly tiring deep into the second, the 36-year-old could surely have been forgiven for taking a rare defeat on the chin and saving himself for Saturday's semi.

Yet, ever the perfectionist, Federer dug deep to run out a 6-7 (5/7) 6-4 6-1 winner ahead of his 14th semi-final in 15 appearances at the ATP Finals. The win also gives Federer more ranking points, putting him in a good position to take the No.1 position in the new year.

The second set was on serve until, at 4-5, Cilic netted a forehand to give Federer a set point which the Swiss converted.

A 100mph forehand service return gave Federer an early break in the decider and Cilic's resistance crumbled quickly as Federer turned on the style to race to victory.

“It was a tough match," Federer said after the mach. "I was in trouble, a set and a break point down, but I found a way out of it and I’m happy to be playing this way.

"I was telling myself to relax a bit, but this one wasn’t the case. It’s nice to play tennis this freely. I started doing that midway through the second set.

"Knowing to be qualified on Tuesday evening is kind of weird, but it gives me a great idea how I want to play on Saturday. I’m looking forward to playing Goffin or Thiem.”

Federer will play the winner of Friday's group-stage match between David Goffin and Dominic Thiem.