TOKYO (Reuters) - World number one Victoria Azarenka pulled out of the Pan Pacific Open citing chronic fatigue, while Maria Sharapova was bundled out by Samantha Stosur as the tournament lost its top two seeds on Thursday.

Samantha Stosur of Australia returns a shot against Maria Sharapova of Russia during their women's singles match at the Pan Pacific Open tennis tournament in Tokyo September 27, 2012. REUTERS/Toru Hanai

French Open champion Sharapova, twice a Tokyo winner, was upset by Australian Stosur 6-4 7-6 in a quarter-final filled with drama and high-quality tennis.

Azarenka withdrew before her quarter-final with Germany's Angelique Kerber after coming into the WTA premier five event complaining of health issues.

The Belarusian, who won this year's Australian Open and reached the final of the U.S. Open earlier this month, had struggled to complete her third round match the previous day.

"Before the tournament I wasn't feeling very good," said Azarenka, who had her blood pressure checked during Wednesday's win over Roberta Vinci.

"I had low energy and I wasn't myself. It wasn't possible today. Maybe it was tiredness from the whole season. It's a little bit unknown what it is."

Stosur next faces Russian Nadia Petrova, who ousted sixth seed Sara Errani 3-6 7-5 6-3 after battling back from a set and 4-1 down against her Italian opponent.

Eighth seed Stosur had won just one of her previous 11 meetings with Sharapova but seized the initiative by taking the first set when the Russian blasted a backhand long.

The Australian was taken to a tiebreak in the second after leading 5-3 but won a nail-biter 12-10 on her fifth match point after a wild forehand from Sharapova.

"When you don't take a few (chances) in a row you don't know what's going to happen," said Stosur, last year's U.S. Open champion. "But I just kept trying to go for it and thankfully it paid off."

(Editing by Peter Rutherford)

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