World Number 2 Maria Sharapova suffered a setback in her preparations for the French Open after she withdrew from the Italian Open because of illness.
Sharapova, who defends her French Open title from 26 May, was due to play Sara Errani in Friday's quarter-finals.
The Russian has been in good form, reaching the Madrid Open final last week only to be beaten by Williams 6-1 6-4.
The 26-year-old comfortably beat American Sloane Stephens 6-2 6-1 on Thursday to reach the last eight in the Italian capital but withdrew due to an illness
''I'm sorry," Sharapova wrote on Twitter. "Didn't recover after being sick last week and it got me again."
Meanwhile, Top seed Serena Williams moved into the semi-finals with a 6-2 6-0 victory over Spain's Carla Suarez Navarro.
It was the American's 22nd victory in a row and means she has overtaken her previous best run of 21 successive wins recorded in 2002 and 2003.
The 15-time Grand Slam winner, who has won her last three tournaments, has only dropped three games in her last two matches as she chases a 51st WTA title.
Serena will next face either two-time Rome winner Jelena Jankovic or Romanian qualifier Simona Halep, who were playing later in Rome.