World number one Victoria Azarenka recovered from a bout of dizziness to beat Italy's Roberta Vinci 6-4 6-2 and reach the quarter-finals of the Pan Pacific Open on Wednesday.
Wrestling again with her service demons, second seed Maria Sharapova produced a laboured 6-2 7-6 victory over Czech Lucie Safarova to also reach the last eight in Tokyo.
Azarenka had her blood pressure checked during the changeover after going 4-3 down in the first set but somehow dug in to take it after Vinci dumped a forehand into the net.
"I felt dizzy on court and my head was spinning between points," said the Belarusian.
"I don't know what exactly it is. Hopefully I'll know as soon as possible and get rid of it. I was pretty lucky. I don't know how I got through really"
World number one Victoria Azarenka
"I was just trying to finish the points as quickly as possible."
Azarenka, who won this year's Australian Open and reached the U.S. Open final earlier this month, found her feet again to race to a 4-0 lead in the second set.
Still looking flushed between points, Azarenka offered her opponent a glimmer of hope by dropping the next two games before slamming the door firmly shut.
A sliced backhand brought business to a quick conclusion, although Azarenka was still grimacing as she shook hands with Vinci and waved to the crowd.
"I arrived a few days before the tournament and really struggled with my health," said Azarenka.
"I've been trying to save my energy for my matches."
"I don't know what exactly it is. Hopefully I'll know as soon as possible and get rid of it. I was pretty lucky. I don't know how I got through really."