Australian Open champions Zhang Shuai and Samantha Stosur made a big comeback in the second set to beat Barbora Krejcikova and Katerina Siniakova 6-3, 7-6 (7) on Thursday, storming into the doubles semifinals of WTA Finals Shenzhen.

Each side came into the clash with on win and one loss in the purple group, but last year's finalists, Czechs Barbora Krejcikova and Katerina Siniakova stood above them in the group on set difference before the game.

Zhang/Stosur played a clutch first set, committing only three unforced errors to nine winners.

Things start out well for the local player and her partner during the second set. The Czech duo converted a triple break point on the way to Zhang/Stosur  dropping their opening service. They would be broken once more as the Czechs built up a 4-0 lead.

Facing a 5-1 deficit in the second set, the veterans came all the way back to seal the match in two sets and and saved a set point in the tiebreak.

"We had a really good beginning in the first set, very aggressive. All of the preparation, all the tactics were implemented. During the match, at the very beginning, there were many points that were very close. Three or four times we could have won the points. We encouraged each other because we knew they were playing really well," Zhang said in the post-match press conference.

"In the second set, we wanted to be as aggressive as we were in the first set. We could have done better. We were also surprised when we were down 1-5. We were able to overturn the score. Our opponents were very strong with very good serves. I think my partner did really, really well today. That is the key to our success," she concluded.

World number one Ashleigh Barty and Belinda Bencic both advanced to the semifinals after contrasting victories on Thursday.

Barty progressed from the round-robin stage with a crushing 6-4, 6-2 win over Petra Kvitova in one hour and 28 minutes. Bencic prevailed 7-5, 1-0 in 61 minutes after late replacement Kiki Bertens retired ill.

Bertens received medical attention after the opening game of the second set, where her blood pressure and pulse were checked, before becoming the second player in the tournament to retire mid-match after Bianca Andreescu withdrew against Karolina Pliskova on Wednesday.

Bencic will play defending champion Elina Svitolina in the semifinal on Saturday.

"I'm so relaxed already. I will just play and fight like always," she said. "She's (Svitolina) one of the players that this court suits a lot more."

Barty, who had already sealed the year-end No.1 ranking, rebounded after a three-set loss to Bertens on Tuesday. She will play the winner of tomorrow's Purple Group match between Wimbledon champion Simona Halep and world number two Pliskova.

"I'm particularly happy with the way I was able to return tonight," Barty said. "I think she was trying to take a little bit more of a risk. That kind of worked in my favor a bit."

(With input from agencies)