Japan's Kei Nishikori suffered a 6-1, 6-4 defeat by Austria's Dominic Thiem at the season-ending ATP Finals on Thursday, ending his hopes of advancing to the semifinals.

World No. 9 Nishikori succumbed to Thiem at the O2 Arena in London after producing an upset over six-time champion Roger Federer on Sunday before losing to South Africa's Kevin Anderson on Tuesday.

"The match was over before I could get a feel for it. Before the tournament, I was playing well but my three matches here were not so good," said Nishikori, who was hoping to reach the ATP Finals semis for the third time in four attempts.

"There were too many unforced errors for me...I think I made it really easy today. He was also playing well."

Needing a win to have any chance of advancing from the round-robin group stage, Nishikori got off to a horror start, dropping his first service game to fall behind 3-0.

He got back on track to make it 3-1 in his second service game, but dropped his next and the set.

Nishikori was more competitive in the second set, but when he missed a chance to break with the score at 2-2, Thiem capitalized, breaking the Japanese before closing the match out on his own serve.

The loss for Nishikori, whose season began with him making a comeback on the Challenger Tour after a long wrist-injury layoff, leaves him with a 43-win, 19-loss record on the ATP Tour and a second straight year without an ATP title.

But when asked to reflect on his season, the 28-year-old was positive, assessing his effort as "really good."

With a straight-sets win over Anderson, Federer topped Nishikori's group, with the South African qualifying in second place.

On the other side of the draw, world No. 1 Novak Djokovic remains unbeaten with one group stage match to play. Either Alexander Zverev or Marin Cilic, both with one win and one loss, will join the Serb in the semifinals.

The Nov. 11-18 event, which features the top eight qualifying singles players and doubles teams, has a round-robin format. The top two in each group progress to the knockout semifinals.