Last four decided in Tokyo
The line-up for the semi-finals of the Toray Pan Pacific Open in Tokyo was finalised on Thursday.
Nadia Petrova, Sam Stosur, Agnieszka Radwanska and Angelique Kerber all booked their places in the last four.
On a thrilling day of quarter-final action, Petrova was the first through, coming back from 6-3 4-1 down to secure a 3-6 7-5 6-3 victory over sixth seed Sara Errani.
It is the world number 18's biggest win in the last three years and as a reward she will come up against Stosur, who made light of her 1-10 record against Maria Sharapova, beating the second seed 6-4 7-6 (12/10).
"I lost nine points in a row from 5-3 up in the second set, but I knew I had to turn it around and not let that continue," Stosur said after the match.
"I had a good start in the tie-break and it really helped me forget about all of that.
Kerber did not have to pick up a racket to go through, with the top-seeded Victoria Azarenka pulling out of their clash with dizziness.
Next opponent Radwanska did have to play, though, seeing off Caroline Wozniacki 6-4 6-3.