Top-ranked Victoria Azarenka of Belarus overcame a dizzy spell to beat Italy's Roberta Vinci 6-4, 6-2 on Wednesday and advance to the quarterfinals of the Pan Pacific Open.
Azarenka, playing in her first tournament since losing in the U.S. Open final to Serena Williams, had her blood pressure checked after going down 4-3 in the first set but bounced back to win the next three games.
"I felt dizzy on the court and my head was spinning in between points which made it difficult to focus," Azarenka said. "What exactly it is, I don't know. I am still trying to figure it out. Hopefully, I'll know as soon as possible and get rid of it."
Azarenka improved to 3-0 in head-to-head matches against the 14th-seeded Vinci and will meet fifth-seeded Angelique Kerber in Thursday's quarterfinals. Kerber defeated Urszula Radwanska of Poland 6-1, 6-1.
Two-time champion Maria Sharapova defeated Lucie Safarova of the Czech Republic 6-2, 7-6 (5) to set up a quarterfinal meeting with eighth-seeded Sam Stosur of Australia, a 6-4, 7-5 winner over Slovakia's Dominika Cibulkova.
The second-seeded Sharapova was coming off a tough, three-hour win over British qualifier Heather Watson on Tuesday, while Safarova advanced to the third round on a walkover.
Sharapova coasted through the first set and led 3-0 before Safarova mounted a comeback, breaking Sharapova three times to take a 6-5 lead.
Sharapova broke Safarova in the final game to force a tiebreaker, which she won with an ace.
"After a long match yesterday I was really happy to win in two sets," Sharapova said. "I had a bit of a letdown in the second set but was really happy that I could close it out in the tiebreaker."
In other matches, Sara Errani of Italy beat Marion Bartoli 3-6, 6-2, 6-2 to set up a quarterfinal showdown with Nadia Petrova of Russia, who beat Petra Martic of Croatia 7-6 (2), 6-7 (1), 6-4.
After struggling in the first set, the sixth-seeded Errani broke Bartoli in the second game of the second set and never looked back.
"It was a tough match on this fast court," said Errani, who has won three titles this year. "I didn't start off well but neither did she. She got a little tired in the second set and that's when I started to play better."
Former No. 1 Caroline Wozniacki of Denmark, seeded 10th, beat China's Li Na 4-6, 6-3, 6-4 and will face third-seeded Agnieszka Radwanska in the quarterfinals. Radwanska beat American player Jamie Hampton 6-4, 6-3.