Novak Djokovic defended Alexander Zverev after the German was booed following a controversial incident during his win over Roger Federer at the ATP Finals.

Zverev booked his spot in the decider in London with a hard-fought 7-5 7-6 (7-5) victory over Federer in their semi-final.

However, the 21-year-old was booed post-match, having stopped playing mid-point during the second-set tie-break, when a ballboy behind Federer dropped a ball which rolled towards the court.

Djokovic, who beat Kevin Anderson to reach the final, joined Federer in defending Zverev.

"I felt bad for Zverev because you could see that he didn't deserve to be booed. I didn't see what he did wrong," the world number one told a news conference.

"It was a really important point, important moment. Roger was on the point and was putting him on the back foot but you could see on the replay that it did actually happen.

"I didn't check the rule exactly that the umpire is the only one that can stop the play if he sees it. I'm not sure, maybe someone can tell me whether the player can do that... That's the question, so nobody knows.

"I think on the moral side, you can't really say anything about Zverev because it did really happen and he's not someone that cheats.

"He's an honest guy and has a really good relationship with players and Roger, so he didn't deserve to get booed, but I understand from a crowd's perspective that it all looked quite odd to happen at that point."

With his win, Zverev became the first German since Boris Becker in 1996 to reach the decider at the season finale.