Roger Federer beats Alexander Zverev, clinches No. 1 seed

LONDON -- Roger Federer returned to a place he's all-too-familiar with, the semifinals of the ATP World Tour Finals.

Federer reached this stage Tuesday in London for the 14th time in 15 attempts with a 7-6 (6), 5-7, 6-1 win against rising star Alexander Zverev.

With Federer up a set and a break, Zverev fought back to win the second, but a series of costly mistakes in the decider gave the Swiss the match and the No. 1 seed in his group.

Federer, a six-time World Tour Finals champ, is still working his way back into peak condition after taking things relatively easy for the past couple of weeks. He was relishing the chance to work on his game with nothing at stake before the semis.

His final round-robin contest in the Boris Becker group will come Thursday against Marin Cilic, who earlier in the day was eliminated from the semifinals by Jack Sock 5-7, 6-2, 7-6 (4).

"I was trying to put a lot of balls into play so there was a lot of defending from me," Federer said after the match. "But it paid off, so I'm happy. I am extremely relieved I can play freely in my last match instead of it being a nail-biter."

Federer's tactical awareness won't need any tweaking, though, as he mostly rode the pressure the big-serving Zverev put on him early in the match.

Federer,36, enjoys this court and the way it plays, and his experience on it against a tournament debutant helped him pass a thorough examination of his game.

He and Zverev, 20, had split their four career head-to-head matches coming into Tuesday's encounter, but the German's inexperience perhaps made the difference down the stretch. Zverev lost his composure after going down an early break in the third and never recovered.

"But, look, all in [all], I think it was a very positive match," Zverev said. "Obviously, he won it.

Zverev is just one ranking spot behind world No. 2 Federer and has been unabashed about trying to unseat him and the rest of the Big Four in the years to come, but his forehand let him down too often on this occasion.

Zverev still has a shot to make the semifinals against another finals debutant, Sock, who became the first American to win a singles match at the event since 2007.

After taking out Cilic, Sock complained about being woken at 4 a.m. local time by a hotel fire alarm.

"It was a tough one for sure," Sock said in his on-court interview. "It has been a long morning."