Sharapova will move to number one in the world
Maria Sharapova came from a set down against Swiss Patty Schnyder to reach the Italian Open semi-finals in Rome.
The 21-year-old Russian won a tough match 6-7 7-5 6-2 to set up a meeting with fourth seed Jelena Jankovic.
There was no place in the last four for either of the Williams sisters. Venus lost 5-7 6-2 6-3 to Jankovic while Serena withdrew because of injury.
Alize Cornet, the player to benefit from Serena's withdrawal, will play Anna Chakvetadze in the semi-finals.
Serena said that her withdrawal would not affect her preparations for the French Open.
"It just happened all of a sudden in practice," said eight-times grand slam winner. "I just went for a shot and then my back got a little stuck.
"I don't expect this to cause any problems with my preparation for the French Open."
Having to pull out of Rome will be a particular disappointment for Serena after she recently put together a 17-match unbeaten run that ended at the German Open.
Frenchwoman Cornet's walkover set up a semi-final with Russian sixth seed Chakvetadze, a 6-2 3-6 6-1 winner over Bulgarian qualifier Tsvetana Pironkova.
Jankovic's win over Venus Williams was her fourth in their seven encounters.
"I've beaten her many times, but I know that I always have to give my best if I want to beat her," said the reigning champion.
"She's really strong and I wanted to attack her as much as possible, put pressure on her and do the right things."
Venus said: "I made a lot of opportunities for myself, but then I made a
mistake. I played an aggressive game. Maybe I went for a little too much, but that's my game."