Federer downs Zverev in 'generation game' to reach semis

A day after the ATP Finals lost Rafael Nadal to injury, Roger Federer lifted spirits at the O2 Arena by extending his stay to the weekend.

The 19-time Grand Slam champion marched into Saturday's semi-finals but he made hard work of the task before finally prevailing 7-6 (8-6), 5-7, 6-1 in a gripping battle of the generations with Germany's Alexander Zverev.

With Novak Djokovic and Andy Murray also absent due to injuries, the top-ranked Nadal's withdrawal left Federer as the only member of men's tennis' ' big-four' in the tournament.

"It was nice to be able to show maybe that quality of mine, that I can dig out these matches, these points time and time again, stay mentally tough," said Federer. "In the third I started to play better. It was a tough match from the beginning till the end."

The 36-year-old six-time champion may have been giving away 16 years to the youngster fancied to be the game's next major force but Federer proved in a dominant third set that, for the moment, he still remains a cut above his young pursuers.

The triumph in two hours and 11 minutes ensured the world number two booked a place in the last four for the 14th time in 15 appearances and with the tournament still only three days old.

"It was a good battle and I always enjoy myself at this beautiful arena," said Federer. "I have so much fun playing in London and I have been so successful here. Thank you for making it such a special night.

"I'm happy and relieved that I can play freely against [Marin] Cilic, instead of it being a nailbiter. It's been a tough group so to be through in two matches is great."

It also meant that Zverev, who won his opening match against Marin Cilic, and Jack Sock will now meet on Thursday to decide the other qualifier from the Boris Becker group.

Sock earlier lived up to his childhood nickname of "Showtime" by producing a dramatic comeback triumph 57, 6-2, 7-6 (7-4) over the erratic Cilic, with Federer's victory eliminating the Croat.

"It was a very positive match," Zverev said. "Obviously he won it. He's in the semifinals. But even for me, I played the better match of the two today. I think I still have great chances of qualifying, playing Jack Sock next. I think if I continue having this level, I don't know, maybe you'll see me on the weekend still."