Madrid, Spain (SportsNetwork.com) - Serena Williams pulled out of the Madrid Open on Friday because of a thigh injury.
Williams was hurt during a first-round win over Belinda Bencic on Sunday, but continued to play and won two more matches in straight sets before deciding to withdraw prior to her scheduled quarterfinal against Petra Kvitova.
"I have a left thigh injury and, unfortunately, have to withdraw from this year's Mutua Madrid Open," Williams said in a statement Friday. "It happened during my first round match. It started to get better, but most importantly right now, I just need some time to rest and recover."
Williams was the top seed and the two-time defending champion at this event.
"It's beyond words. It's so frustrating," Williams added. "We couldn't ask for a better tournament and I love it here in Madrid. I love being a champion here. This is not the way I wanted this week to end."
Williams is still planning to play next week in Rome.
Kvitova, meanwhile, received a walkover into the semifinals.