Serena Williams has undergone surgery on her big toes and withdrawn from an exhibition match against top-ranked Victoria Azarenka in Thailand this month.

A medical certificate from Florida podiatrist Jeffrey Rockefeller says Williams "was treated for a chronic foot disorder which involved minor procedures on both of her great toes."

She needed to minimize activity to fully recover, he added.

Williams, the WTA player of the year after winning Wimbledon, the U.S. Open and London Olympics, was replaced by Li Na for the exhibition on Dec. 29 in Hua Hin.