Kaia Kanepi eliminated top-ranked Caroline Wozniacki 7-5, 1-6, 6-4 on Wednesday to advance to the quarterfinals of the Pan Pacific Open.
The 43rd-ranked Kanepi rallied from three games down in the third set and won the match when she broke the defending champion with a forehand down the line.
After a sluggish start, Wozniacki dominated the second set but was unable to keep the momentum in the third when she was broken three times by the Estonian player.
"I played some good tennis in the second set and the first half of the third but then I couldn't get my serve in," Wozniacki said. "Kaia played a good match today. I'll try to do better next time."
Kanepi will play the winner of a later match between Agnieszka Radwanska and Jelena Jankovic in Thursday's quarterfinals.
In other matches, second-seeded Maria Sharapova earned a hard-fought 7-6 (4), 7-6 (4) win over 13th-seeded Julia Goerges of Germany.
Sharapova came back from a 3-1 deficit in the first set and then rallied from 5-3 down in the second.
"It was a tough match," Sharapova said. "Two sets go to tiebreak, I was pleased that although I really didn't play my best in the match I stepped it up in the tiebreaks and that was the difference."
Sharapova will face fifth-seeded Petra Kvitova of the Czech Republic on Thursday in a rematch of this year's Wimbledon final. Kvitova beat Vania King of the United States 6-1, 7-6 (4).
"It was a tough loss against her at Wimbledon," Sharapova said. "I thought she played great tennis. She hit the ball big and deep, had a great serve. I hope I can learn from that match and I'm looking forward to it."
Maria Kirilenko of Russia also advanced with a 6-3, 6-1 win over Ana Ivanovic and seventh-seeded Marion Bartoli of France coasted to a 6-2, 6-1 win over Peng Shuai of China.
Kirilenko, who knocked out U.S. Open champion Samantha Stosur on Tuesday, broke her opponent in the first game of the second set and never looked threatened by the 12th-ranked Serb, who took a medical time-out midway through the second set because of a sore back.
"To be honest, I was a little bit tired today when I woke up," Kirilenko said. "I didn't have that much time to recover because we finished so late yesterday."
Kirilenko broke Ivanovic again to go up 4-1 in the second set and took advantage of her opponent's sixth double fault in the last game to close out the match in 1 hour, 19 minutes at Ariake Colosseum.
Kirilenko will next face compatriot and fourth-seeded Vera Zvonareva of Russia, who defeated Iveta Benesova of the Czech Republic 6-1, 6-2.
"You do get more and more confident with each win," said Kirilenko, who has yet to beat Zvonareva in five matches. "I've had so many tough matches against Vera. Hopefully, tomorrow will be another tough one."
Third-seeded Victoria Azarenka of Belarus defeated Klara Zakopalova of the Czech Republic, 6-3, 7-5 and will face Bartoli on Thursday.