Sharapova, a two-time winner and the world number two, had been due to play Italy's Sara Errani in the quarter-finals.

However the Russian announced her withdrawal on Friday morning citing sickness and claiming she had run a temperature overnight.

Sharapova does not have a habit of withdrawing from tournaments but said she already felt under par in Madrid last week and did not want her heavy schedule compromising her chances in Paris.

"I've a lot on my schedule and they all mean a lot for me and obviously with the French Open coming up it is important to make smart decisions," said Sharapova.

Sharapova said she felt physically under par during her win over Sloane Stevens on Thursday, but that she had also been "a bit off" in Madrid last week.

"You could see it in my energy last night," she said. "I was already a bit off in Madrid and played through it and it came back last night and I got sick again...

"It's not smart to again compete two or three rounds to win the tournament and with Paris starting next week."

Asked whether the fact that it was her boyfriend's birthday on Thursday was a factor, she replied: "Is that a question or a comment? I don't think that has anything to do with it."

Top seed Serena Williams, meanwhile, sailed through to her semi-final with a straight sets win over Spain's Carla Suarez Navarro.

And the American said she could understand Sharapova's decision given the scheduling of Rome between the Madrid and French Opens.

"It's definitely taking a chance and sometimes you might feel like preparing well for the Roland Garros, which is probably the most difficult Slam to win, and you want to give yourself the best chance and sometimes this is what you have to do."

Errani, the world number six, will now go through to the semi-finals to meet the winner of the match between Australia's Samantha Stosur and Viktoria Azarenka of Belarus.