Australian world number one Ashleigh Barty reached the semi-finals of the WTA Finals with a straight-set win over Czech sixth seed Petra Kvitova.

Barty, 23, won 6-4 6-2 to eliminate the two-time Wimbledon champion from the season-ending event in Shenzhen.

Swiss seventh seed Belinda Bencic also progressed after Kiki Bertens retired from their match through illness.

Bencic, who was leading 7-5 1-0, will face defending champion Elina Svitolina in the semi-finals.

Dutch alternate Bertens, who replaced the injured Naomi Osaka earlier this week, is the third player to withdraw from the eight-player field, with US Open champion Bianca Andreescu pulling out with a knee injury earlier on Thursday.

French Open champion Barty, competing in the WTA Finals singles for the first time after a stunning year, recovered from Tuesday's defeat by Bertens with a dominant win over an out-of-sorts Kvitova.

Barty edged the key moments in the opening set, breaking for a 3-2 lead and saving break points in each of her next two service games, then ran away with the second as Kvitova began to tire.

"I felt I executed really well and knew I had to come out and play aggressively," said Barty, who has won her past three matches against Kvitova.

"I had a few early break points against me and I played them really well. It was important to keep my nose in front."

Ukraine's Svitolina was the first player to make the last four by beating Wimbledon champion Simona Halep in the Purple Group on Wednesday.

Romanian Halep plays Czech Karolina Pliskova in their final round-robin match on Friday, with Barty awaiting the winner in Saturday's semi-finals.