Venus Williams snatched victory from the jaws of defeat to beat fast-rising Romanian Simona Halep 4-6, 7-5, 6-3 and reach the quarter-finals of the Pan Pacific Open on Wednesday.

The 33-year-old American, who dominated women's tennis a little over a decade ago but whose career has been blighted since being diagnosed with the autoimmune disorder Sjogren's Syndrome in 2011, warned after her match in Tokyo that she had no thoughts of retiring.

In any case, sister Serena would not let her, Venus said.

"I don't know how I was able to win that match," Venus told reporters. "She played unbelievable. I've never seen her hit the ball that hard.

"It came down to who was more determined, who would fight harder and believe more. I'm glad that person was me."

Williams dropped the first set and was down 4-3 and a break in the second before producing an impressive comeback to set up a meeting with Canada's Eugenie Bouchard, who upset sixth seed Jelena Jankovic 7-5, 6-2 under a closed centre-court roof as rain halted play outside.

Only Serena, with nine, has won more titles than Halep's four in 2013 and the 13th seed gave Venus the runaround for a set and a half, leaving the seven-times grand slam champion shaking her head as she whipped winners from all parts of the court.

But Williams, who has slipped to 63rd in the world after two years of illness or injury, showed her experience on the big points and, after levelling at a set-all, ruined Halep's hopes of celebrating her 22nd birthday on Friday still alive in the tournament with a blistering crosscourt forehand on her first match point.

Williams added: "The motivation for me now is knowing my luck's got to change.

"My body's got to cooperate at some point. What am I going to do anyway? Watch on TV? That's just lazy.

"And Serena wouldn't let me quit," she added with a laugh.

"She'd be like, 'You're playing doubles!'. I'd be, 'Okay'.

"I perhaps don't have the health of the other players but physically I'm always trying to get an edge.

"I've tried traditional medicine and non-pharmaceutical things. It's like tennis -- you always have to try new things.

"There's no roadmap. I'm on my own here."

Denmark's Caroline Wozniacki, winner in Tokyo three years ago, swept into the last eight with a 6-1, 6-1 demolition of Slovakian Magdalena Rybarikova.