Serena Williams has withdrawn from the Western & Southern Open after aggravating a right toe injury.

The 31st-ranked Williams, who won her opening match in straight sets on Tuesday, was scheduled to play 10th-seeded Samantha Stosur. Williams is coming back from injuries and illness that kept her sidelined for much of the past year.

The win Tuesday was Williams's 12th straight, tied for the longest on the WTA Tour this year. She was seeking her third consecutive tournament championship.

Stosur advances to the round of 16 and will face French Open champion and fifth-seeded Li Na on Thursday.