- Madrid Open
Williams withdraws with thigh injury
Serena Williams has pulled out of the Madrid Open, citing a left thigh injury.
The world No.1, who was a double defending champion in the Spanish capital, says she is "planning to play" at the Rome Masters, which starts on Sunday.
"I have a bye so have an extra day of rest," she said. "I will take it one day at a time."
On the injury, she added: "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. 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."
While Williams remains optimistic she will play in Italy, the problem raises concerns that she will not make the second grand slam of the year - the French Open - which gets underway on May 25.
Williams' withdrawal means quarter-final opponent Petra Kvitova is handed a bye to the last four, where she will face fourth seed Simona Halep, who needed precisely one hour to get past former world No.1 Ana Ivanovic 6-2 6-2.
On the other side of the draw Maria Sharapova came from a set down to beat second seed Li Na 2-6 7-6(5) 6-3.
Li was in control until she failed to convert two break points in the 11th game of the second set. Errors then began creeping into the Australian Open champion's game as Sharapova evened the match.
After an exchange of breaks in the third, Sharapova, who lost to Williams in last year's final, broke again and then held serve to win on Li's 43rd unforced error.
Sharapova will face Agnieszka Radwanska for a place in the final after the third seed edged French qualifier Caroline Garcia 6-4 4-6 6-4 in their quarter-final match.