* World number one overcomes dizzy spell
* Sharapova through despite erratic display
* China's Li toppled by Wozniacki (Recasts after Azarenka win)
By Alastair Himmer
TOKYO, Sept 26 (Reuters) - 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 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."
French Open champion Sharapova finished with an ace but double-faults on big points almost cost the Russian the second set as the sun set over Tokyo Bay.
"After a long match yesterday, I was just happy to win in two (sets)," said Sharapova, who had taken more than three hours to dispatch British qualifier Heather Watson the previous day.
The two-times Pan Pacific Open champion was almost forced into a decider having blown a 3-0 lead in the set before winning the tiebreak 7-5.
Sharapova almost choked in the breaker too, opening a 4-1 advantage before a wild forehand allowed Safarova to drew level, prompting a furious reaction from the Russian.
Muttering to herself angrily between points, Sharapova smacked her racquet against the court in disgust but held her nerve and had just enough in the tank to get the job done.
"After being up 3-0 I was frustrated," said the Olympic silver medallist, who served 14 double faults in her opener and six against Safarova.
"I don't think it should have gone to a tiebreak because I had the momentum going in the second set."
Sharapova next faces eighth seed Samantha Stosur of Australia in the WTA premier five event after last year's U.S. Open champion beat Slovakia's Dominika Cibulkova 6-4 7-5.
Seventh seed Li Na, one of three Chinese players to travel to Tokyo despite escalating political tensions between China and Japan, was beaten 4-6 6-3 6-4 by Denmark's Caroline Wozniacki.
Former world number one Wozniacki next faces defending champion Agnieszka Radwanska, the third seed, who progressed with a 6-4 6-3 win over American qualifier Jamie Hampton.
Fifth seed Angelique Kerber played party pooper for the Radwanska family, however, overpowering Agnieszka's younger sister Urszula 6-1 6-1. Azarenka awaits the German.
Sixth seed Sara Errani ended the French resistance, overcoming ninth seed Marion Bartoli 3-6 6-2 6-2 to reach the last eight.
Reporting by Alastair Himmer; Editing by John O'Brien and Martyn Herman