Tokyo, Japan (Sports Network) - Victoria Azarenka withdrew from her scheduled quarterfinal match at the Pan Pacific Open on Thursday because of dizziness and Maria Sharapova was beaten in straight sets, leaving the top two seeds out of the lucrative tournament.
The top-seeded Azarenka was slated to play Angelique Kerber, but instead gave the fifth-seeded German a walkover into the semifinals, where she will play defending champion Agnieszka Radwanska. The third-seeded Pole advanced Thursday with a 6-4, 6-3 victory over former world No. 1 Caroline Wozniacki.
Australia's Samantha Stosur knocked off the second-seeded Sharapova, 6-4, 7-6 (12-10), and earned a semifinal matchup with Russian Nadia Petrova, who rallied for a 3-6, 7-5, 6-3 victory over Italy's Sara Errani.
Stosur, this week's eighth seed, beat Sharapova for just the second time in 11 all-time meetings. The lone previous win came last year during the WTA Championships. She is trying for her first title since capturing the 2011 U.S. Open for her maiden Grand Slam crown. Her best finish this year was a loss to Azarenka in the Doha final back in February.
Sharapova, who saved four match points on Thursday before finally wilting, won this title in 2005 and 2009.
Petrova didn't face a match point Thursday, but was down 6-3, 4-1 and had just double faulted to face another break point before storming back. She'll take a 5-3 lifetime mark against Stosur into Friday's semifinals. Stosur, however, has won three of the last four meetings.
Radwanska broke serve five times and had her serve broken just twice in topping Wozniacki for only the third time in eight all-time meetings. Wozniacki, the 2010 Pan Pacific champ, was bidding for consecutive titles after claiming her first of the year last week in Seoul.
A three-time winner already this year and runner-up at Wimbledon, Radwanska was in line for yet another matchup with Azarenka before the Belarus native pulled out Thursday.
Azarenka and Radwanska had already met six times this year -- all between January and May -- and Azarenka had won all six meetings. Radwanska's last win in the series came in the semifinals of this event last year.
Instead, Radwanska will carry a 3-2 mark against Kerber into the semis. Radwanska has won the last two meetings, including a 6-3, 6-4 triumph in the Wimbledon semifinals this summer and a three-set victory in the second round of this tournament last year.