Defending champion Serena Williams pulled out of her Madrid Open quarter-final against Petra Kvitova as a precautionary measure on Friday due to a left thigh injury.

The withdrawal of Williams, who has landed the title in each of the last two years, handed Kvitova a walkover into the semi-finals, where she will face Simona Halep or Ana Ivanovic.

With just over two weeks until the start of the French Open on May 25, Williams was determined she should not put her place at Roland Garros at any further risk by allowing the problem to worsen.

She said on the tournament website: "I have an injury in my left thigh, and unfortunately I'll have to retire from this year's Mutua Madrid Open.

"I suffered it during my first-round match. I started to feel better but it is important that I have enough time to rest and recover."

Williams had beaten Swiss player Belinda Bencic in the first round, before wins over China's Shuai Peng and Spain's Carla Suarez Navarro carried her through to the last eight.

"It's very frustrating," she said of her setback. "This isn't the way I wanted to end the week."