Defending champion Novak Djokovic recovered from a shaky start to beat Dominic Thiem in the first match of the ATP World Tour Finals on Sunday.

The 12-time grand slam champion is aiming to return to the top of the world rankings in London after his 122-week reign was ended by Andy Murray at the Paris Masters and battled back to defeat Thiem 6-7 (10-12) 6-0 6-2.

Thiem had never taken a set off the Serbian in their previous meetings, but came out on top in a marathon tie-break to take the opener before the second seed hit back in emphatic fashion in Group Ivan Lendl.

Djokovic, winner of the season-ending tournament in each of the last four years and five times in all, needed treatment for a cut thumb in the first set and was warned for unsportsmanlike conduct after Thiem deservedly won a tense breaker.

Umpire Carlos Bernardes issued the warning to the Australian and French Open champion when he hit a ball towards his player's box after going a set down.

That was as good as it got for eighth seed Thiem, as Djokovic harnessed his frustration by sweeping the tournament debutant aside in clinical fashion with a second-set bagel and two breaks in the decider to seal victory in just over two hours.