World number one Ashleigh Barty booked her place in the semi-finals of the WTA Finals with a comprehensive 6-4 6-2 victory over Petra Kvitova in Shenzhen.

Barty's win puts her into the last four as group winner with two wins from three round-robin matches.

The Australian was disappointed to lose against Kiki Bertens on Tuesday from a strong position but made no mistake this time, surging into a 4-0 lead in the second set and saving all four break points she faced.

We need your consent to load this YouTube contentWe use YouTube to manage extra content that can set cookies on your device and collect data about your activity. Please review their details and accept them to load the content.Manage Preferences

The tournament suffered its second withdrawal on Thursday when Bianca Andreescu pulled out.

The US Open champion suffered a knee injury early in her second match against Karolina Pliskova on Wednesday and was forced to retire after the first set.

Andreescu said: "I had a scan Thursday to check my knee after last night's match, and unfortunately the results show that I need to withdraw. 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."

American Sofia Kenin will take Andreescu's place, making her debut at the tournament against Elina Svitolina on Friday.

There was another retirement in the second singles match of the day, with Bertens pulling the plug when Belinda Bencic held a 7-5 1-0 advantage.

Bertens came in as an alternate after Naomi Osaka's withdrawal and defeated Barty but a heavy schedule appears to have taken its toll.

Bencic progresses to the last four on her debut.