Novak Djokovic allowed his frustration to spill over but was ultimately satisfied with his opening 6-7 (10/12) 6-0 6-2 win over ATP World Tour Finals debutant Dominic Thiem at London's O2 Arena.

Playing his first match since losing the world number one ranking to Andy Murray last Monday, Djokovic had to battle back from a set down.

The turning point came at the end of the first set when, having seen Thiem take his seventh chance to win the tie-break, Djokovic angrily smashed a ball in the direction of his support camp.

It flew into the stands, but, fortunately for Djokovic, did not hit anyone in the sell-out crowd.

Djokovic faced questions about possibly being defaulted at the French Open this summer when he bounced his racket in annoyance and almost hit a line judge.

The 29-year-old reacted angrily to a similar line of inquiry here, responding to the suggestion that it could have been a serious incident by saying: "It could have been, yes. It could have snowed in O2 Arena as well, but it didn't.

"It is not an issue for me. It's not the first time I did it."

Djokovic has lost only one match here in four years, winning the title on each occasion, but he will need to turn his form around to continue that run.

Struggles with motivation, an unspecified personal issue as well as wrist and arm problems have resulted in Djokovic winning just one tournament since he completed his set of grand slam titles at the French Open in June.

As the holder of all four slams, the Serbian appeared unstoppable, but five months is a long time in tennis.

His 122-week run as world number one has come to an end and Djokovic must out-perform Murray here to reclaim the position.

The Serbian's normally fearsome groundstrokes carried little pop as he and Thiem kept each other at arm's length in the opening set.

Djokovic's imperious backhand has been a particular problem in recent months and he was choosing to rush into the net rather than prolong rallies.

Thiem thumped a backhand winner down the line to open up a 6-3 lead in the tie-break only to double fault twice.

Four more set points went begging, but on Thiem's seventh opening he took advantage of passive play from his opponent to win his first set in four matches against Djokovic.

However, that was as good as it got for the 23-year-old, with Djokovic racing through the second set in just 23 minutes and winning 12 of the last 14 games.

"It felt very good," said Djokovic. "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.

"I knew after that the first couple of 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. Obviously I made him play an extra shot. 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."