MASON, OHIO. - Red-hot Serena Williams pulled out of the draw on Wednesday, while world No. 1 Caroline Wozniacki went by way of a second- round upset at the $2.05 million Western & Southern Open, a U.S. Open Series event.

The former world No. 1 Williams was scheduled to meet Aussie Samantha Stosur in a rematch of their championship match clash this past Sunday in Toronto, but the oft-injured American decided against playing after aggravating a right toe injury.

The 10th-seeded Stosur thus received a walkover into Thursday's third round on the hardcourts at the Lindner Family Tennis Center. Stosur will meet French Open champion Li Na on Day 4.

The 31st-ranked 13-time Grand Slam singles champion Williams is riding a 12- match winning streak, including back-to-back titles in Stanford and Toronto. She beat Stosur in straight sets in the finale in Canada and was a straight- set winner in her opening match here in Mason on Tuesday. Her right foot was a little swollen following the victory over Czech Lucie Hradecka on Tuesday.

Williams returned to action back in June after being sidelined for 11 months due to injuries and illness.

A cautious Williams knows that the 2011 U.S. Open gets underway in less than two weeks.

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

The former U.S. Open runner-up Wozniacki, meanwhile, was stunned by promising American teenager Christina McHale 6-4, 7-5 and dropped her second straight tournament-opening match. The wild card McHale is a 19-year-old from Englewood Cliffs, New Jersey.

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

Second-seeded 2010 U.S. Open runner-up Vera Zvonareva moved on with a 6-3, 6-0 pasting of fellow Russian Ekaterina Makarova. Zvonareva captured this tournament in 2006 and was the runner-up here in 2004.

Belarus' Victoria Azarenka, the third seed, withdrew from her match with Israel's Shahar Peer because of a strained right hand. She was replaced by lucky loser Pauline Parmentier of France, who dropped the match with a 6-2, 6-3 defeat.

Seventh-seeded French Open runner-up Francesca Schiavone held off Russian glamour girl Maria Kirilenko 7-5, 3-6, 6-4, while 13th-seeded former No. 1 Jelena Jankovic fought back to beat Chinese qualifier Zheng Jie 4-6, 6-3, 6-1, and two-time major titlist Svetlana Kuznetsova overcame Czech qualifier Petra Cetkovska 4-6, 6-3, 6-4. Jankovic was the Cincinnati champion two years ago. The former top-five Russian star Kuznetsova, seeded 14th this week, will face Wimbledon runner-up and fellow Russian Maria Sharapova on Thursday. The former world No. 1 and three-time major champion Sharapova was last year's Cincy runner-up to U.S. Open champ Kim Clijsters.

Former top-10 Russian Nadia Petrova bested 15th-seeded former No. 1 Ana Ivanovic of Serbia 6-3, 7-6 (7-4), while a mild upset came when Croatian qualifier Petra Martic took out 17th-seeded Belgian Yanina Wickmayer 3-6, 6-2, 6-2. Sixteenth-seeded Chinese Peng Shuai avoided an upset by handling Italian Sara Errani 6-2, 6-4, while Slovakian veteran Daniela Hantuchova humbled fellow non-seed Flavia Pennetta of Italy 6-3, 6-4.

In other second-round action, ninth-seeded German Andrea Petkovic cruised to a 6-2, 6-4 victory over Swede Sofia Arvidsson. Petkovic, a quarterfinalist in the Australian Open, fell behind 2-4 in the second set before rallying for the victory.

Four current or former world No. 1s were on the Wednesday schedule.

The 2011 Cincinnati champion will collect $360,000.