Alexander Zverev apologised in the face of heckles from the O2 Arena crowd for a controversial incident in his 7-5 7-6 (7-5) ATP Finals victory over Roger Federer.

Zverev became the first German since Boris Becker in 1996 to reach the final but drew the ire of sections of the auditorium when he stopped playing mid-point during the second-set tie-break.

A ballboy behind Federer dropped a ball which rolled towards the court, the eventually replayed point going the way of Zverev, who went on to see out the match.

Zverev, 21, was booed as he started his on-court interview by some who deemed him to have gained an unfair advantage and he immediately moved to apologise for the incident.

"First of all I want to apologise for the situation in the tie-break," he said. "The ballboy dropped the ball and it's in the rules that we have to replay the point."

Interviewer Annabel Croft also reminded the crowd of the rules and urged spectators to be "a little bit more respectful" to Zverev, who played superbly and will face Novak Djokovic or Kevin Anderson in the final.

Zverev continued: "I apologised to Roger at the net already and he said it's okay, that's obviously in the rules like that."

"I want to apologise to the crowd as well. I know there are a lot of Roger fans here. As he deserves, he should [have a lot of fans]."

"For what he's achieved and the kind of guy he is, he should have the most fans in the world. And obviously here in London, especially, with how much history he has here."

"The crowd has been amazing and fair the whole match. I'm very sorry that this happened. I didn't mean to upset anybody. That's all I can say. Sorry."

Tournament director Adam Hogg later tweeted: "Astonishingly disappointing reaction from the crowd.  Huge credit to Sascha Zverev for reaching his first final here at the ATP Finals. And well done Annabel Croft on handling the situation with aplomb."