Novak Djokovic expressed frustration after his on-court conduct was questioned in the wake of a victory over Dominic Thiem at the ATP World Tour Finals.

Djokovic – playing his first tournament since losing the number-one ranking to Andy Murray – recovered impressively from losing the opening set to beat Thiem 6-7 (10-12) 6-0 6-2 at London's O2 Arena on Sunday.

When the Serbian subsequently addressed the media, he appeared less than impressed to have been asked about an incident at the end of the first set, in which he fired a ball towards his player box and was warned by the chair umpire.

A reporter asked Djokovic: "Does it concern you one day that will cost you dearly? If you hit someone ..."

"You guys are unbelievable," came the reply. "You're always picking these kind 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."

When it was suggested that the ball could have hit a spectator and caused a serious incident, Djokovic sarcastically responded: "It could have been, yes. It could have snowed in the O2 Arena as well, but it didn't."

Prior to the frosty exchange, the reigning World Tour Finals champion had reflected on his triumph over Thiem.

"Even though I lost the first set, I thought I didn't do too many things wrong. It was just the very high quality of his game that prevailed in the first set," said Djokovic.

"I knew after that, the first games of the second set would be crucial for me to start with a break up, which I did. I felt more comfortable. I started swinging more freely in the second set. He started making more errors, which I used.

"I was on top of his second serves, putting a lot of pressure. I thought I played very well in the second set especially, but the third as well."