By Martyn Herman

LONDON (Reuters) - Deposed world number one Novak Djokovic suffered a scare at the hands of Austrian debutant Dominic Thiem before launching his bid for a fifth successive ATP World Tour Finals title with a 6-7(10) 6-0 6-2 victory on Sunday.

Knocked off the summit he had occupied for 122 weeks by Andy Murray last week, Djokovic must better his British rival's performance at London's O2 Arena to snatch back the top ranking, but things did not go entirely to plan.

Thiem, the youngest of the eight qualifiers for the year-ender at 23, grew increasingly dangerous throughout the first set and blazed several stunning winners to pocket a gripping breaker on his seventh set point.

After that, though, the match was a procession as Djokovic racked up a 19th win in his last 20 matches at the tournament.

Even after sealing victory though, Djokovic's mood was soured by a reporter asking him about an angry reaction to losing the opening set when the Serb slammed a ball into the ground and it bounced into the stands, prompting a warning from the umpire.

When it was suggested he could have found himself in hot water had the ball hit a spectator, a tetchy Djokovic snapped: "It could have been yeah, but it could have snowed in the O2 Arena as well, but it didn't."

Hiccups aside, including a cut finger that required treatment, the 12-times grand slam champion was happy to get a victory that now turns the spotlight on Murray who plays Marin Cilic in his opening match on Monday.

Djokovic, bidding to end the year ranked number one for the fifth time in six seasons, trails Murray by 405 points in the standings but Murray will lose the 275 points he gained for winning the Davis Cup for Britain last year.

That means they are virtually neck-and-neck and the 200 points Djokovic earned for beating Thiem edged him ahead in what is turning into a thrilling race for the year-end honor.

Djokovic's Ivan Lendl group looks easier than Murray's section. As well as Thiem, the Serb will face Canada's Wimbledon runner-up Milos Raonic and Gael Monfils.

He has never lost a match to any of them although Raonic was in ominously good form as he overpowered Monfils 6-3 6-4 in Sunday's late match -- offering not a single break point in a dominant display.

Murray takes on former U.S. Open champion Cilic in the John McEnroe group on Monday and also faces reigning U.S. Open champion Stan Wawrinka and Japan's Kei Nishikori, who beat the Scot at Flushing Meadows this year.

Without doing anything spectacular Djokovic was cruising throughout the first set against Thiem, dropping only four points on serve on the way to a 5-4 lead.

Thiem was not overawed though and forced Djokovic to save a break point at 5-5.

The tiebreaker was a classic. Three sublime winners helped Thiem into a 6-3 lead but he twice double-faulted as Djokovic clawed back to 6-6.

Thiem had three further set points to Djokovic's one before nailing a forehand winner at 11-10 to huge roars from the sell-out crowd in the arena.

Djokovic, whose grip on men's tennis has been prised loose since he won the French Open in June, seized on a Thiem wobble to run away with the second set in 23 minutes.

While Thiem offered more resistance in the third there was only going to be one winner once Djokovic broke to lead 2-1.

"I knew the opening couple of games of the second set would be crucial for me," Djokovic said.

"I started swinging more freely."

(Reporting by Martyn Herman, editing by Pritha Sarkar)