Summary Serena had worn a heavy strapping on her left leg in straight sets victories over Belinda.
MADRID, May 09, 2014 (AFP) - World number one Serena Williams pulled out of her quarter-final at the Madrid Open against Petra Kvitova on Friday morning due to a thigh injury but has confirmed she will play in Rome next week.
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.
