LONDON, Nov. 17 (Xinhua) -- Alexander Zverev became the youngest player to reach the final of the ATP Finals since 2009 and the first German to do so since Boris Becker in 1996.

Zverev ended Roger Federer's hopes of clinching his 100th singles title this season with a thrilling 7-5, 7-6(5) over the Swiss in London on Saturday afternoon.

The 21-year-old will now meet world number one Novak Djokovic after the Serbian defeated Kevin Anderson 6-2, 6-2 in the other semifinal.

Federer went into the game with a 3-2 record against Zverev and had already beaten him in the 'round-robin' stage of the tournament.

But this time the youngster wasn't willing to give an inch as the duo swapped groundstrokes and the Zverev claimed the set when he broke Federer to love in the 12th game, when most would have been expecting a tiebreak.

The second set was another close affair with both sides trading blows and this time it did go to a tiebreak, which ended in controversy, when a ball boy dropped a ball during a rally and Zverev asked for the point to be replayed.

Sections of the crowd booed him for the request, and he made reference to the incident in his post-game interview with Sky Sports TV.

"Firstly I want to apologize for the situation in the tie-break but the ball boy dropped the ball, it's in the rules that we have to replay the point."

"I apologized to Roger (Federer) at the net, he told me 'it's okay, it's in the rules'. I want to apologize to the crowd, Roger has a lot of fans here and for what he's achieved he should have. I'm very sorry that this happened I didn't mean to upset anybody - that's all I can say, sorry," said the German.