Federer enters Semi

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 guaranteed his place in Saturday's semifinals by defeating Alexander Zverev 7-6 (6), 5-7, 6-1 on Tuesday.

With Novak Djokovic and Andy Murray also absent due to injuries, the top-ranked Nadal's withdrawal on Monday 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," Federer said. "In the third I started to play better. It was a tough match from the beginning till the end."

Zverev, who won his opening match against Marin Cilic, will face Jack Sock on Thursday to decide who will join Federer in the final four. Sock defeated Cilic in the early match, 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."

After an opening set dominated by serve went to a tiebreak, Federer came from 4-0 down and saved a set point to take the lead. His grip on the match quickly tightened as a distracted Zverev made two double faults to gift a break in the opening game of the second set.

However, the 20-year-old regained his poise and took advantage of a dip in Federer's first serve percentage to break back and then repeated the feat to force a deciding set.

From there on it was all Federer. The six-time champion, who is 16-years older than his opponent, showed his big-match experience as he regrouped to break in the third game, before doing so twice more to seal the victory.

Sock's 5-7, 6-2, 7-6 (4) victory was the United States' first at the event in 10 years.

The No 8-seed, who seized qualification by winning the final Masters event of the season in Paris, lost his debut match to Federer, but he is now 1-1.

While others might have been surprised at Sock becoming the first American to win at the elite season-ending event since Andy Roddick in 2007, the man himself wasn't.

"I wouldn't be out here playing, competing if I didn't think I could give myself a chance to win tournaments, to be here," Sock said.

Dimitrov reaches s/f

Grigor Dimitrov advanced to the semifinals at the ATP Finals by beating David Goffin 6-0, 6-2 Wednesday.

Dimitrov, who is 2-0 at the season-ending tournament, lost only two points on his serve in the first set and twice broke Goffin without even going to deuce.

Goffin finally managed to hold serve in the second set, but Dimitrov still was able to break the Belgian's serve two more times.

The sixth-seeded Bulgarian will next face Pablo Carreno Busta, who entered the tournament after Rafael Nadal withdrew because of a right knee injury.