Top seed Caroline Wozniacki reached the Pan Pacific Open semi-finals after Polish opponent Agnieszka Radwanska retired with a foot injury on Thursday.
The Dane was leading 5-0 in the first set when sixth seed Radwanska called for the trainer at the changeover and decided to pull out of the $2 million tournament in Tokyo.
Wozniacki, winner of five titles this year and closing in on the world number one ranking, plays eighth seed Victoria Azarenka of Belarus in the semi-finals.
Azarenka beat American qualifier Coco Vandeweghe 6-2 6-1.
"I've been playing some great tennis," Wozniacki told reporters. "I am sad for Agnieszka of course but I''m happy with my performance.
"It's always been a goal of mine to be number one," added Wozniacki, who could leapfrog Serena Williams if she wins this week and reaches the last eight in Beijing next week.
"But if it doesn''t happen this year there will be other chances."
French Open champion Francesca Schiavone of Italy beat Estonia's Kaia Kanepi 7-5 4-6 7-5 and will face either Elena Dementieva or Vera Zvonareva for a place in Sunday's final.
