This guarantees Serena Williams the No. 1 ranking going into the French Open.
Because she could not defend her title in Rome, Sharapova will lose points and fall further behind Williams.
After winning the Italian Open in 2011 and 2012, Sharapova must now focus on defending her French Open title later this month.
Sharapova apologized to her fans via Twitter. She said she was still recovering from an illness that she came down with in Madrid.
Maria Sharapova @MariaSharapova
I'm sorry for having to withdraw from the tournament this morning. Didn't recover after being sick last week and it got me again last night.2013-5-17 10:20:53
It’s not clear what illness Sharapova is suffering from. She appeared fine on Thursday night in her victory in straight sets over Sloane Stephens, 6-2, 6-1.
Meanwhile, Williams won her quarterfinals match, 6-2, 6-0, over Carla Suarez Navarro. It was the 22nd straight victory for Williams, a personal best, according to the Associated Press.
After she won the French Open last year, some in the media crowned Sharapova the “Queen of Clay." However, Sharapova has yet to defeat Williams on that surface.
Until she does, her clay-court crown will remain suspect—especially since Williams is undefeated on clay this season.
Sharapova will leave Rome knowing that her best chance to snatch the No. 1 ranking from Williams was probably last week in Madrid. The Russian entered Madrid riding a 17-match winning streak on clay. Williams had not won on red clay in more than a decade, according to Peter Bodo of ESPN.com.
Separated by fewer than 600 points, they entered the Madrid final with the No. 1 ranking on the line. Williams defeated Sharapova, 6-1, 6-4. Williams now holds a 13-2 record against Sharapova.
Depending on how she performs at Roland Garros, Sharapova’s next best chance to reclaim the top spot will be this summer. Williams has to defend her Wimbledon, Stanford and U.S. Open titles.
Williams has no points to defend at the French Open because she suffered a stunning first-round defeat last year.
Going into a match, Williams likes to say she has nothing to lose. Having defended her points in Charleston, Madrid and Rome, Williams literally has nothing to lose at the French Open this time.
Follow Merlisa on Twitter: @merlisa