Serena Williams has withdrawn from the WTA Tour green clay tournament in Charleston next week, along with Russian Maria Sharapova and defending champion Sabine Lisicki, organizers said.
Williams said in a statement released by the tournament that the knee injury that has sidelined her since her Australian Open triumph "has not fully healed" and that she looks forward to "returning to the WTA Tour in Europe."
Lisicki is out because of a left ankle injury. Sharapova announced her withdrawl as she continues to recover from a right elbow injury that hindered her at the Indian Wells hardcourt tournament last month and prompted her to withdraw from the Miami hardcourt event that ended last weekend.
She was diagnosed with a bone bruise.
The clay-court season will pick up steam in Europe later this month, building to the French Open at Roland Garros, which starts on May 23.