Serena withdraws from Madrid but says will play Rome
Serena Williams of the US returns the ball to Spain's Carla Suarez Navarro during their Madrid Masters third round singles' match at the Magic Box (Caja Magica) sports complex in Madrid on May 8, 2014 - by Pierre-Philippe Marcou
The 17-time Grand Slam champion had worn a heavy strapping on her left leg in straight sets victories over Belinda Bencic, Peng Shuai and Carla Suarez Navarro earlier in the week.
"I have a left thigh injury and, unfortunately, have to withdraw from this year's Madrid Open," said Williams.
"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.
"It's beyond words. It's so frustrating."
Initially the tournament organisers in Madrid had also ruled her out for next week's Internazionale BNL D'Italia in Rome, but the American insisted she will try to defend her title in the Italian capital.
"I am planning to play in Rome. Fortunately, I have a bye next week so will have an extra day of rest.
"For now, I will take it one day at a time and take advantage of some extra rest."
Her withdrawal is just the latest setback to the tournament after two of the top four seeds in the men's Masters -- Novak Djokovic and Roger Federer -- also withdrew due to injury and family reasons respectively.
Meanwhile, fifth seed Kvitova will face Simona Halep or Ana Ivanovic in the last four on Saturday.