Federer stuck on 99

Alexander Zverev beat Roger Federer 7-5, 7-6 (7/5) on Saturday to reach the title decider at the ATP Finals in a match that ended in confusion after a ball boy's fumble forced a point to be replayed.

The 21-year-old German is now just one win away from the biggest title of his career but the Swiss must wait for a tilt at his 100th tournament victory.

The end of the match was overshadowed by an unusual incident, Zverev stopping a rally with Federer leading 4-3 in the second-set tie-break after the ball boy dropped a ball. Zverev won the replayed point with an ace and nudged into a 5-4 lead before Federer inexplicably netted with a simple forehand volley to hand the German two set points, the second of which he took on his own serve.

Immediately after the match finished there were boos from the pro-Federer crowd at London's O2 Arena and on-court interviewer Annabel Croft told them to be "more respectful" as Zverev had been playing by the rules.

"First of all I want to apologise for the situation in the tie-break," said the German. "The ball boy dropped the ball and it is the rules we have to stop the point."

Federer described it as a "big call" but said Zverev was not to blame.

"It was obviously a big call. Instead of being in the rally in a decent position, you get aced, yes, it makes a difference," he said. "It could have made a difference. That's all hypothetically speaking now at this point."

In Saturday's later match, world number one Novak Djokovic takes on South Africa's Kevin Anderson in a repeat of the Wimbledon final for the right to face the German in Sunday's final.