The Serb, who is trying to regain the world number one ranking from Britain's Andy Murray, won 6-7 (10-12) 6-0 6-2.

Djokovic was warned for unsportsmanlike conduct after losing the first set but won 12 of the last 14 games.

If he wins at least two group matches and goes on to lift the trophy he will return to the top of the rankings.

Scot Murray, who replaced Djokovic as number one last week, plays Marin Cilic in his opening match in the John McEnroe Group on Monday.

"I thought I played very well in the second set especially, but the third as well," said Djokovic.

"When I had the great comeback of saving six, seven set points, then I didn't manage to win that first set, of course you're frustrated.

"On the other hand, I think I managed to kind of compose myself and really gather all my attention and concentration to what was coming up after that."

Milos Raonic broke Gael Monfils once in each set to win 6-3 6-4 in the second Ivan Lendl Group match, after Britain's Jamie Murray and Brazilian partner Bruno Soares beat Treat Huey and Max Mirnyi in the doubles.

Big-serving Canadian Raonic will play Djokovic in his next match on Tuesday, while Frenchman Monfils takes on Thiem.

Angry outburst 'not an issue'

Five-time champion Djokovic went into Sunday's match with doubts surrounding his form after a disappointing second half of the season, but grew stronger as the contest wore on.

Thiem needed seven set points but eventually secured the opener, with Djokovic looking uncertain after getting early treatment on a cut finger.

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The Serb's frustration boiled over when he lost a gripping tie-break, and he fired a ball in the direction of his player box which narrowly missed coaches Boris Becker and Marian Vajda.

It drew a warning from umpire Carlos Bernardes but appeared to release rather than increase the tension felt by Djokovic, who eased away from debutant Thiem.

After a 67-minute first set, Djokovic needed just 55 minutes to race through sets two and three, roaring with delight after breaking serve in the third.

Asked whether he was concerned that another flash of temper could risk a default, Djokovic said it was "not an issue for me".

When it was suggested that ball he batted away in anger could easily have hit a fan or official, he added: "It could have snowed in O2 Arena, as well, but it didn't."