Milos Raonic showed no sign of the leg injury that put his place in the ATP World Tour Finals at risk when he beat debutant Gael Monfils handsomely at the O2 Arena on Sunday.

The Canadian pulled out of his Paris Masters semi-final against Andy Murray with a right thigh injury earlier this month and it was touch and go whether he recovered in time for the London year-ender.

Wimbledon runner-up Raonic made a flying start, though, with a 6-3 6-4 Ivan Lendl Group victory over flamboyant Frenchman Monfils.

It was his first victory at the O2 after his debut two years ago resulted in two defeats before he withdrew injured.

Raonic offered up no break points and pummelled the ball with his hefty groundstrokes to keep Monfils on the run.

The world No. 4 moved surprisingly smoothly although he said later that there were still some concerns.

"Maybe I hesitated a little bit a few times especially on the longer runs," Raonic told reporters.

"I hope I can get rid of those hesitations as the days go on. I feel like this is an ideal way to start considering the doubts that I was having.

"Some of the things I heard in the diagnosis were not the most positive so this was a great way to turn it around."

Providing Raonic is 100 per cent fit, the 25-year-old will be a formidable opponent for defending champion Novak Djokovic when they clash on Tuesday.

He is unlikely to run out of energy either thanks to the dried fruit snacks he was nibbling during the changeovers.

"They were dates," said Raonic. "I don't like them but apparently they are very good for sustaining energy. You can see from how small a bit I take that I really don't enjoy them."  

2nd-ranked Serb survives early setback

Novak Djokovic survived an early setback, and then made it look all too easy.

The second-ranked Serb, who has a chance to reclaim the No. 1 ranking at the O2 Arena next weekend, rallied to beat Dominic Thiem 6-7 (10), 6-0, 6-2 in his opening match. 

Djokovic takes 1st step in defending ATP Tour Finals crown0:44

Djokovic won nine of 10 games to take control of the match after losing the first set in a tiebreaker. And even that was close.

"Yeah, a thrilling tiebreaker," Djokovic said. "I had, I think, only one set point. He just played a good point. I was in the rally, but he just was going for his shots."

Thiem had his first three set points at 6-3 in the tiebreaker, but he double-faulted twice and then put a backhand into the net to make it 6-6.

Djokovic had a chance, too, leading 9-8. But he couldn't close it out with Thiem serving, eventually hitting a backhand long. The Austrian finally won it on his seventh set point with a forehand winner, prompting Djokovic to smack a ball into the crowd.

Djokovic takes over

After that, it was just about all Djokovic. The Serb reeled off six straight games to send it to a third set, and then broke Thiem twice more to close it out.

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

In the following sets, it got to be too much for the Austrian, who was making his debut at the tournament for the top eight players in the world.

"I lost a little bit energy, not much, but just a little bit (after the first set)," Thiem said. "Of course, against a player like Novak, he immediately steps up. That's how the second set went."

Thiem meets Mourinho 

Thiem said he had a chance to meet Manchester United manager Jose Mourinho after the match, and the pair chatted briefly. Thiem is a fan of English club Chelsea, a team Mourinho has coached in the past.

Djokovic lost the No. 1 ranking to Andy Murray last week, but he can earn it right back in London. If either of the two wins the title next Sunday, that one will be assured of the top ranking — as long as Djokovic wins another round-robin match on the way to the final.

If Djokovic does reclaim the No. 1 ranking, he would finish the season at the top for the third season in a row and the fifth time in six years. Murray is trying to end up as the top man for the first time.

With a victory next Sunday, Djokovic can also join Roger Federer as the only players to win the season-ending tournament six times. Federer is not playing in London, sitting out the rest of the season as he continues to recover from knee surgery.

With files from the Associated Press