Kei Nishikori bowed out of the ATP Finals with a whimper just four days after his shock win over Roger Federer.

The Japanese world number nine looked poised to end the season on a high after taking the scalp of six-time winner Federer in their opening round-robin match at London's 02 Arena.

But in the next three sets he played, Nishikori won just two games as he was pulverised 6-0 6-1 by Kevin Anderson on Tuesday, before Dominic Thiem won Thursday afternoon's opener 6-1.

Nishikori smashed his racket on the ground in disgust but he did at least make a better fist of the second set, although he was powerless to prevent Thiem running out 6-1 6-4 winner.

"I felt fresh, but I just didn't feel the ball this week," said Nishikori.

"It could be the court or the ball. I don't know. But, yeah, I just didn't play well this week.

"I mean, I was same in the last match, too, but there were too many unforced errors for me.

"I think I made it really easy today. I mean, he was also playing well but I think I gave him too many free points."

Thiem, the Austrian world number eight, was relieved to break his duck at this year's Finals following straight-sets defeats by Anderson and Federer.

"I just went into the match and wanted to show my real self again, basically, because two days ago (against Federer) was really bad," said the French Open runner-up.

"I was not feeling good to play such a match in front of such a big crowd and everything.

"I wanted to have a much better game again today. That's what I did."