Novak Djokovic reacted sharply to questions about a ball-smashing incident during his opening match at the ATP Tour Finals, saying he received too much scrutiny.

Playing Dominic Thiem, the Serb dropped the first set of the match in a tiebreaker and smashed a ball that went towards the stands. He came back to win the next two sets for a 6-7 (10), 6-0, 6-2 victory.

"You guys are unbelievable," he responded when asked if he was concerned about a penalty for such incidents. "You're always picking these kinds of things. I'm the only player that shows his frustration on the court? That's what you are saying?... It's not an issue for me. It's not the first time I did it."

Djokovic agreed that it could have been significant if he had hit a spectator.

"It could have snowed in The O2 Arena as well, but it didn't,” he said, however.

He also disagreed with suggestions that he was displaying such behavior more often, and pointed to his improved play during the rest of the encounter.

"I had made a great comeback, saving six or seven set points, and then when I didn’t manage to win that first set, of course you’re frustrated,” he said. “On the other hand, I think I managed to compose myself and really gather all my attention and concentrate [on] what was coming up after that. The great start of the second set helped me.”

Djokovic is the second seed at the ATP Tour Finals. He’ll play Milos Raonic on Tuesday.