TOKY - Former world number one Venus Williams overpowered ailing top seed Victoria Azarenka 6-2 6-4 with ruthless efficiency to reach the last 16 of the Pan Pacific Open on Tuesday.

The seven-time grand slam champion showed glimpses of her gunslinging best under the Tokyo floodlights against a grimacing Azerenka, who spent much of the match bending over her racquet between points and tugging sharply on her purple top in frustration.

"I've sat through two US Open finals between her and Serena and you could tell from the first game she wasn't herself," Williams told reporters. "It was definitely hard to concentrate because you sort of feel bad. I don't know what was bothering her but I just hope she feels better." Azarenka, beaten by Serena Williams in the US Open final earlier this month in a repeat of the 2012 championship match in New York, had a large fan brought courtside to try to cool off at changeovers.

But such was her obvious discomfort she failed to threaten Williams. The 24-year-old Belarusian threw several tantrums as she repeatedly wafted shots off target or into the net. "I haven't been able to practise for three days and I didn't sleep at all last night," said Azarenka. "I felt hot, then cold. Maybe I ate something bad but it's been getting worse," added the world number two, who complained of having eaten some bad sushi after a quarter-final exit from the $2.3 million event last year. "You can't play at 20 percent against a top player -- it just doesn't work that way."

Williams completed her third victory in three meetings between the pair with a second serve. Australian Open champion Azarenka wearily hooked it wide before trudging off court close to tears. "I'm getting better every tournament," said Williams, currently ranked a lowly 63rd in the world after two years of illness and injury problems. "I've had my issues, to say the least, but it's given me an unreal amount of character." Big-hitting Australian Samantha Stosur, seeded 12th, shunted out Japan's golden oldie Kimiko Date-Krumm, although the 2011 US Open champion had to work for her 6-3 7-6 win over the 42-year-old local favourite. "I hung in as best I could but she played aggressively on the big points and didn't really show any weaknesses," said the 63rd-ranked Date-Krumm, who reached a career-high fourth in the world back in 1995.  "I'm disappointed to lose but I moved pretty well so the glass is half full."  Russia's Svetlana Kuznetsova, twice a grand slam singles champion, upset third seed Sara Errani of Italy, registering a 6-4 6-4 win to reach the third round. Sixth seed Jelena Jankovic brushed aside Japan's Ayumi Morita 6-4 6-1 to advance. Another Serbian former world number one, Ana Ivanovic -- runner-up to Martina Hingis in Tokyo in 2007 -- thrashed Ukraine's Elina Svitolina 6-3 6-0 to set up a meeting with fifth seed Angelique Kerber of Germany.

Canada's Eugenie Bouchard pulled off a surprise 5-7 7-6 6-3 victory over ninth-seeded Sloane Stephens, whose fellow American Madison Keys fared far better with a mature display in a 7-5 6-2 victory over China's Peng Shuai that belied her tender age of 18.