Red-hot Serena Williams pulled out of the draw 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.

Wozniacki, meanwhile, was stunned by promising 19-year-old American wild card Christina McHale 6-4, 7-5 and dropped her second straight tournament-opening match.

Seventh-seeded French Open runner-up Francesca Schiavone held off Russian glamour girl Maria Kirilenko 7-5, 3-6, 6-4, while two-time major titlist Svetlana Kuznetsova overcame Czech qualifier Petra Cetkovska 4-6, 6-3, 6-4. The former top-five Russian star Kuznetsova is seeded 14th here.

A mild upset came when Croatian qualifier Petra Martic took out 17th-seeded Belgian Yanina Wickmayer 3-6, 6-2, 6-2.

The 2011 Cincinnati champion will collect $360,000.