Top-ranked Ash Barty reached the semifinals in her debut appearance at the WTA Finals, beating Petra Kvitova 6-4, 6-2 Thursday to finish with a 2-1 record in the Red Group.

Barty, the first Australian woman to earn year-end No. 1 ranking, won her first Grand Slam trophy at the French Open in June and then reached the top of the rankings.

"I feel like I executed really well tonight," Barty said on court. "Overall, I knew I had to come out here and play aggressively, and play to win.

"I'm really excited to have another chance to come out and play here on this beautiful court."

Belinda Bencic joined Barty in the semifinals after Kiki Bertens retired in their match with the Swiss player leading 7-5, 1-0.

Bencic, like Barty, is making her WTA Finals singles debut.

"It's pretty amazing, especially (because) I made it @ the last minute," said Bencic, the last to qualify for the eight-player field. "I'm through the group (even though) I lost the first match here."

Barty will next play either Karolina Pliskova or Simona Halep in the semifinals, while Bencic will face defending champion Elina Svitolina.

Barty saved all four break points she faced in the first set and broke Kvitova's serve in the fifth game. The Czech player posted 20 unforced errors in the opening set.

In the first game of the second set, Barty broke Kvitova's serve on her third break point and quickly jumped out to a 4-0 lead.

Kvitova, who won the season-ending title in her debut in 2011, finished the round robin 0-3 for the second consecutive year.

"When it came to the deciding points, she always played better than me," Kvitova said. "I think the serve didn't help me at all today."

Bertens entered the tournament as an alternate after Naomi Osaka withdrew on Tuesday with a right shoulder injury.

The Dutchwoman, who would have moved onto the semifinals if she had beaten Bencic in the match, had her blood pressure taken on the court. She was teary-eyed when she decided to stop.

"I didn't feel well on court," Bertens said. "I had some problems with my stomach. The energy was not there and I was a little bit dizzy. It was just too much to continue."

Also Thursday, Bianca Andreescu withdrew from the tournament with a left knee injury. She will be replaced by Sofia Kenin for the final round robin match on Friday against Svitolina.

"I had a scan Thursday to check my knee after last night's match, and unfortunately the results show that I need to withdraw," Andreescu said. "I'm very disappointed to not be able to finish the year on my terms, but I am hopeful I will have more chances to play here in the future."

Kenin, who was guaranteed $125,000 as an alternate, will now earn at least $165,000 as a participant. If she wins her match against Svitolina, she will earn an additional $305,000.

Prior to Kenin's inclusion, it was the first women's year-end final in the 49-year history of the tournament not to feature an American in either singles or doubles.