Top-seeded Caroline Wozniacki advanced to the Pan Pacific Open semifinals when Agnieszka Radwanska retired with a left foot injury in the first set.
At 5-0 down in Thursday's match, the sixth-seeded Radwanska consulted with a trainer during the changeover and the Pole decided she could not continue.
Wozniacki, aiming for her sixth title this year, will meet eighth-seeded Victoria Azarenka of Belarus in the semis on Friday. Azarenka defeated American qualifier Coco Vandeweghe 6-2, 6-1.
''I'm happy about my performance, I feel I have played some great tennis here,'' Wozniacki said. ''Of course, I am sad for Agnieszka and hopefully she will be back soon.''
The Dane could overtake Serena Williams at No. 1 in the world rankings if she wins this tournament and reaches the quarterfinals in Beijing next week.
''If I reach No. 1 that is great,'' Wozniacki said. ''It's always been a goal of mine but if it doesn't happen this year there will be other chances.''
In other matches, French Open champion Francesca Schiavone beat Kaia Kanepi of Estonia 7-5, 4-6, 7-5.
The Fifth-seeded Italian went ahead 6-5 in the third set on a double-fault by Kanepi. She clinched the match with an unreturnable serve to the corner.
Schiavone will meet the winner of the quarterfinal between Elena Dementieva and Vera Zvonareva.
