08/17/2011 01:41 EDT | Updated 10/17/2011 05:12 EDT

Serena withdraws from Cincinnati with sore toe, No. 1 Caroline Wozniacki upset

MASON, Ohio - Serena Williams woke up with a sore and swollen right big toe on Wednesday, prompting her to withdraw from the Western & Southern Open — the latest in her long streak of foot problems and setbacks.

Williams had won two straight tournaments at Stanford and Toronto, and her win on Tuesday night at the Cincinnati-area tournament was her seventh match in eight days.

She decided to withdraw and rest the foot, which she cut on glass at a restaurant and needed two operations to repair last year. Williams was rounding back into form with the U.S. Open less than two weeks away.

"I don't think this is a good time for me to take a big chance," she said. "I just don't think that would be smart."

Williams is the third high-profile player to miss some or all of the tournament because of injury. Sister Venus Williams withdrew before the start because of a virus that also forced her to sit out the Rogers Cup in Toronto. Defending champion Kim Clijsters couldn't play because of an injured abdomen.

The tournament also lost top-ranked Caroline Wozniacki on Wednesday. Wozniacki dropped her opening match for the second consecutive week, falling to American Christina McHale 6-4, 7-5 in the second round.

In the men's bracket, second-seeded Rafael Nadal beat qualifier Julien Benneteau 6-4, 7-5 despite burns on the index and middle finger of his right hand, which the left-hander uses to steady his backhand shot. Nadal said he touched a hot plate at a restaurant a few days ago. Both fingers were wrapped in gauze for protection.

"The plate was very hot and I got burned," he said.

Serena Williams was trying to win three in a row for the first time since the start of the 2008 season. She's won 12 straight matches, equalling the longest such streak on the WTA Tour this year.

And, she seemed to be beyond the foot and other problems that dropped her to No. 175 in the rankings. Her back-to-back tournament titles moved her all the way up to No. 31.

She beat Lucie Hradecka in two sets on Tuesday night. She said the toe was sore during the match, but not so much that it was a concern.

When she awoke Wednesday, it was sore and swollen. She talked to her father and her sister, then decided to withdraw and rest for a few days.

"It's not as bad as it was before," she said. "It's just a lot of match play. I've been playing a lot of the matches, more than I have in a long time, and also training as well. It got a little aggravated. Instead of making it worse, I've decided it will be in my best health not to go on."

The time off also will give her the opportunity to attend the wedding Kim Kardashian, who is marrying NBA player Kris Humphries in California over the weekend. Williams is a friend of the reality TV star.

"Now that I have time, I probably will," she said. "I hadn't thought about it."

After winning her fourth singles title at Wimbledon in 2010, Williams cut her foot on glass at a restaurant, starting a streak of injuries and ailments that kept her away from tennis for nearly a year. Besides the two operations on her foot, she was diagnosed in February with blood clots in her lung. Then, she needed treatment for a pocket of blood that formed under the skin on her stomach.

She wasn't overly concerned about the latest foot problem coming before the U.S. Open.

"What scares me is if I keep going and not listening to my body," she said. "Right now, I'm really in tune and really in touch with that. We have a great relationship."

While Williams was deciding to withdraw, Wozniacki was struggling to keep the ball in play. She also lost her opening match last week in Toronto.

The Dane had never faced the 76th-ranked McHale, who broke Wozniacki in the 11th game of the second set with a delicate drop shot. She clinched the match when Wozniacki sailed a forehand long.

"I can't remember the last time I had two first-round exits," she said. "It's a bit different. You know, I usually play better as the week goes on, but what happens, happens."