Last year's runner-up Agnieszka Radwanska suffered a quarterfinal upset, while former world No. 1 star Venus Williams reached the semifinals at the $2.37 million Pan Pacific Open tennis event.
Fifth-seeded German left-hander Angelique Kerber doused the second-seeded Radwanska 6-4, 6-4 on the hardcourts at Ariake Coliseum. Last year's Wimbledon runner-up and the 2011 Tokyo titlist Radwanska was fresh off her title in Seoul last week.
Radwanska lost to Russian Nadia Petrova in last year's finale in Tokyo.
Meanwhile, the 33-year-old Williams landed in the final four by holding off rising 19-year-old Canadian and fellow non-seed Eugenie Bouchard 6-3, 6-7 (4-7), 6-3 in three hours on Day 5.
"She played so well," Williams said. "She's going to be a great player -- she's already a great player -- and it was just a fight to the end.
"I was just fighting until the end. It was disappointing to lose the second set, but to win the third set is the goal, so I'm happy."
The surprising Williams stunned top-seeded and fellow former No. 1 Victoria Azarenka on Tuesday.
Two other quarterfinals saw fourth-seeded former world No. 1 Caroline Wozniacki come from behind to beat Czech Lucie Safarova 2-6, 6-3, 6-2 and seventh-seeded Petra Kvitova handle two-time Grand Slam champion Svetlana Kuznetsova 6-4, 6-1.
The third round also had to be completed on Thursday, as the former Wimbledon champion Kvitova whipped American Madison Keys 6-2, 6-2 and the former top- five star and 2008 Tokyo finalist Kuznetsova upended 15th-seeded Romanian Sorana Cirstea 7-6 (7-3), 6-1.
Friday's semifinals will pit the 2010 Pan Pacific champion Wozniacki against Kerber and Kvitova versus her fellow former Wimbledon champ Williams.
The 2013 Pan Pacific titlist will earn $426,000.