Serena Williams has pulled out of the Madrid Open due to a thigh injury.
The world No 1 withdrew ahead of her quarter-final against Petra Kvitova on Friday morning due to a thigh injury.
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.
But now Williams, who won in Madrid for the past two years, faces a race to be fit to defend her title in Rome next week as she continues her preparations for the French Open.
"I have a left thigh injury and, unfortunately, have to withdraw from this year's Madrid Open," Williams said in a statement.
"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...
"This is not the way I wanted this week to end."
Meanwhile, fifth seed Kvitova will face Simona Halep or Ana Ivanovic in the last four on Saturday.