Austria's Dominic Thiem kept alive his hopes of reaching the semifinals of the ATP Tour Finals with a 6-3, 1-6, 6-4 victory over Gael Monfils on Tuesday.

Thiem lost his opening group stage match against Novak Djokovic, but the Tour Finals debutant could still make the last four after seeing off flamboyant Frenchman Monfils at London's O2 Arena.

The 23-year-old will need to defeat Wimbledon finalist Milos Raonic in his last group match on Thursday to reach the semi-finals.

Monfils is certain to be eliminated after losing his first two matches at the prestigious season-ending event.

"It was a very close match but luckily he helped me in the last game. Maybe I was the lucky one today," Thiem said.

"I am very happy there's still a chance for me to reach the semi-finals, but Milos Raonic is an amazing opponent.

"I can't wait to get back out here in this amazing arena."

Thiem is regarded as an emerging star after a breakthrough season and the Austrian had shown no signs of being intimidated by the Tour Finals stage when he took a set off defending champion Djokovic.

His gutsy effort and aggressive stroke-play also earned a famous fan in Manchester United manager Jose Mourinho, who met Thiem after the match and gave him an autograph.

Thiem should garner plenty more backers around the world if he continues to develop at his current rapid pace.

Thiem was too powerful for the error-prone Monfils in the first set, securing the only break in the sixth game and using his thudding serve to stay ahead.

But, after seeming to be hampered by injury in the first set, Monfils suddenly found his rhythm in a one-sided second set, while Thiem lost his way.

Monfils broke twice in the first four games with some eye-catching groundstrokes, including a majestic winner down the line from an improbable position wide behind the baseline.

Thiem folded in similar fashion against Djokovic, but he was far more focused in the final set against Monfils and clinched the win with a break in the ninth game after a string of double faults from the Frenchman.