Victoria Azarenka overcame a dizzying spell to power past Roberta Vinci 6-4, 6-2 and enter the quarterfinals of the Pan Pacific Open in Tokyo.
The world number one, who lost in a thrilling match to Serena Williams in the final grand Slam of the year at the U.S. Open, called for the trainer midway through the first set, and after some treatment -- including a check of her blood pressure -- the Belarusian got back her bearing and eased to victory. She will now take on Angelique Kerber in the last-eight after the German defeated Urszula Radwanska 6-1, 6-1.
"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."
There were also wins for Maria Sharapova, Agnieszka Radwanska, Caroline Wozniacki, Samantha Stosur, Sara Errani and Nadia Petrova.
Sharapova, after being tested by Heather Watson in the previous round, again struggled with her serves, eventually pulling it together in the second set tiebreak to post a 6-2, 7-6 (5) win over Lucie Safarova.
"After the long match yesterday, I'm very happy to be able to win today," the Russian said. "I was a little bit down in the second set and I'm just happy to win in two sets."
Third seed Radwanska will face former world number one Wozniacki after they got through their respective matches. The Pole beat Jamie Hampton of the U.S. 6-4, 6-3, while Wozniacki fought past Li Na 4-6, 6-3, 6-4.
"After the first set of my first match, my energy level dropped, but since then I've been feeling great," Wozniacki said.
"Today Li Na was playing very close to the baseline and pushing me away, taking the balls very early -- she was all over me. But in the second set I really started to step in and push her away."
This year's surprise player Errani continued her good form by getting the better of Marion Bartoli 3-6, 6-2, 6-2.
"She played very well in the first set but the first few games of the second set were very tight, and I think she was a bit more tired after those -- I started to play better from there," Errani said. "I was trying to play a lot of different shots, like some high balls then some short balls too, and I think I made it difficult for her.
"Today it was important to play my own game. She hits very hard, and if you start to do the same it's very difficult because she plays that game very well."
