Martina Hingis had a better day, winning in her return from ankle surgery. She beat Magui Serna 6-4, 6-3 in a second-round match.

Williams first injured her knee during practice Monday and aggravated the injury during practice Tuesday.

``I took a nap this afternoon and when I woke up, my knee was swollen,'' Williams said. ``I have to take a few days off and try to get ready for the U.S. Open, that's my main goal now.''

The French Open and Wimbledon champion was scheduled to play her first-round match against Fabiola Zuluaga of Colombia on Tuesday.

Williams' spot in the 56-player draw was taken by American Melien Tu.

The sixth-seeded Hingis, who had an operation to repair ligament damage in her left ankle May 20, made several unforced errors but still got past No. 53-ranked Serna.

``It was a great first match and I'm happy to go through,'' Hingis said.

Hingis underwent two ankle operations in less than a year. In addition to her surgery in May, the former No. 1 player had surgery to repair three torn ligaments in her right ankle in October.

Hingis, who won this event twice, extended her win streak at the Canadian tournament to 11 matches. She missed last year's tournament due to injury.

In other matches, American qualifier Laura Granville beat Arantxa Sanchez Vicario 6-2, 6-2; and Patty Schnyder of Switzerland beat Russian Elena Likhovtseva 7-6 (4), 7-5.