Agnieszka Radwanska, fresh off her title at the Sony Ericsson Open in Miami, has withdrawn from the Family Circle Cup due to a back injury.

Radwanska beat Russian Maria Sharapova in straight sets on Saturday for her second title of 2012 and the ninth of her career. But the 23-year-old native of Poland will not be able to compete at the event in Charleston, which starts on Monday.

"I'm very sorry that I can't play in Charleston this year. I played there a few years ago and was really looking forward to returning," said Radwanska. "I know it is a great event and a beautiful city, so hopefully I can come back and play there next year."

Because of Radwanska's withdrawal at the upcoming clay court tournament, third seed Marion Bartoli of France was moved into the No. 1 seeding. American Serena Williams moved up two slots to the third seed.