* Top six seeds out in Tokyo
* Sharapova powers into last 16
* Wozniacki retires after five games
(Adds Wozniacki retirement, Sharapova quotes)
By Alastair Himmer
TOKYO, Sept 29 (Reuters) - Russia's Elena Dementieva became the latest high-profile casualty at the Pan Pacific Open when the third seed was upset by Ukraine's Kateryna Bondarenko in the second round on Tuesday.
The Olympic champion was beaten 6-2 6-7 6-1 to leave the $2 million Tokyo event shorn of its top three seeds and she was quickly joined at the checkout counter by fourth seeded Dane Caroline Wozniacki, who retired with a viral infection.
Former world number one Maria Sharapova had no such problems, however, powering into the last 16 in Tokyo with an almost flawless 6-0 6-1 victory over Australia's Samantha Stosur.
Current number one Dinara Safina, second seed Venus Williams and French Open champion Svetlana Kuznetsova all lost their opening matches on Monday after first-round byes.
A tournament which began with nine of the world's top 10 was down to just two before the close of play on Tuesday.
Dementieva blamed some questionable line calls and fatigue for her loss to a player she had not conceded a set to in four previous meetings.
"There were way too many bad calls from the chair umpire today," she grumbled to reporters. "I'm surprised they don't have a challenge system at such a big event."
Dementieva, a second-round loser at the U.S. Open earlier this month, added: "I played way too many matches in the U.S. Open series. I just didn't have the energy."
Sharapova, 25th in the world rankings following shoulder surgery a year ago, answered the prayers of tournament organisers by overpowering 12th seed Stosur in just 57 minutes.
Stosur at least avoided the dreaded "double bagel" by saving two match points at 0-5 in the second set but the Russian wrapped up a clinical victory with an ace two minutes later.
"I just want to finish the year on a good note," Sharapova said. "I served smart. It was definitely a good day at the office all around."
Sharapova next faces countrywoman Alisa Kleybanova, who knocked out sixth seed Vera Zvonareva 3-6 6-4 6-2 in another Russian clash.
Beaten U.S. Open finalist Wozniacki walked off court in obvious discomfort after losing the first five games of her match against Canada's Aleksandra Wozniak. (Editing by John O'Brien; To query or comment on this story email firstname.lastname@example.org)