Injured Monfils taken out by Thiem in London

— -- LONDON -- Sprawled out in pain on the court, Gael Monfils gave a reminder of the brutal nature of the ATP Tour and the way the game has evolved.

At the top level, players have to be extremely fit just to win a match these days, and Monfils wasn't. He was beaten 6-3, 1-6, 6-4 by Dominic Thiem in Tuesday's afternoon session at the World Tour Finals.

"Not my best," was the Frenchman's blunt assessment of his fitness out there.

Monfils came into London with only three days' practice, having skipped the Paris Masters to ensure he could be here for his debut Tour Finals.

He admitted he sitll felt sore on the evening of the tournament, as well as after his opening defeat to Milos Raonic in London.

Often players' do not have enough time to fully recover between matches, and small niggles become worse every day until they develop into a full-blown problem.

Thiem went a set up in just 25 minutes here, with Monfils winning just five points on the Austrian's serve.

The 30-year-old was hobbling around in pain with an apparent knee injury, toiling at the baseline and unable to slide and chase down what the Austrian was throwing at him to full effect.

He did race into a 4-0 lead in the second set, however, forcing the decider with Thiem threatening to crumble again like he did after winning the opening set against Novak Djokovic Sunday.

But a Monfils double fault gave Thiem match point in what had been a close-fought third set, and a long forehand ended the contest.

"I give my best, as usual, fighting with everything I had," he added. "That's the only matters when you step on the court."

After a second defeat in two matches, it is fair to wonder whether Monfils will play his final round robin match Thursday against Djokovic, with alternate David Goffin waiting in the wings.

"I will try to recover because obviously I'm not good enough to play back-to-back. So hopefully, you know, I'll be much better," said Monfils.

With his first Tour Finals victory, Thiem kept his hopes of making the semifinals alive. He next faces Raonic Thursday.

"Very happy that when I'm facing him that there's still something going on," Thiem said in an on-court interview. "Potentially I have the chance to reach the semifinals. Just try to get fit again for the last match and go.

"It was a very close match [against Monfils]. I had a much better start. He was playing good second set. Third set was very tight, and he helped me a little bit in the last game of the match. Maybe I was a little bit the lucky one today."

The win also guarantees Thiem a top-eight finish this year, with Rafael Nadal bumped down to ninth place in the rankings.