Serena Williams withdrew from the Acura Classic in California on Friday as she was suffering from an inflammation to her left knee.

"Serena is scheduled to have an MRI scan later on Friday but she still expects to play in Montreal next week," said a WTA spokeswoman.

The injury was on the same knee Serena had surgery on last August. The six-times Grand Slam champion was sidelined for eight months as she recuperated from the injury and returned to the women's tour in March.

"I knew that it really hurt but I was hoping it would go away," Serena said on Friday. "I don't want to take the chance of missing out on the Olympics and the U.S. Open.

"I actually did (injured) it just before the anniversary of my surgery. Maybe that's bad memories. But I'm sure it's going to be okay."

The 22-year-old has not won a tournament since the Nasdaq-100 Open in Miami in April.

Despite the WTA's reassurances that Serena would play in Montreal, the American expressed her doubts.

"I think the WTA would hate if I answered honestly," she said. "I'll be vague. It's almost as if people don't care if you play hurt. It's all about making money. I'm about making money. That's what it is all about."

She felt it would be worse for the tour if she played in Montreal and injured the knee further, which would then rule her our for the next three or four months.

Top seed Serena was scheduled to play Russia's Vera Zvonareva in the quarter-finals on Friday.

Zvonareva, seeded 12th, will now receive a walkover and will next play either Anastasia Myskina or Wimbledon champion Maria Sharapova for a place in the final.

The Olympics runs from August 13-29 in Athens while the U.S. Open begins in New York on August 30.