Maria Sharapova of Russia celebrates winning a point against Li Na of China during their women's singles quarterfinal match at the Madrid Open tennis tournament May 9, 2014. [Photo/Agencies]

Li Na of China serves the ball to Maria Sharapova of Russia during their women's singles quarterfinal match at the Madrid Open tennis tournament May 9, 2014. [Photo/Agencies]

MADRID - Maria Sharapova rallied to earn a spot in the semifinals of the Madrid Open on Friday, a few hours after defending champion Serena Williams withdrew from her match because of injury.

Sharapova, who lost to Williams in last year's final, beat Li Na 2-6, 7-6 (5), 6-3 on clay at the Magic Box.

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 set, Sharapova broke again and then held serve to win on Li's 43rd unforced error.

Williams, the two-time defending champion, withdrew because of a left thigh injury.