Serena Williams' Roland Garros hopes in doubt after Italian Open withdrawal

Serena Williams has pulled out of the Italian Open due to a persistent knee injury, casting doubt on her participation in the French Open, which begins later this month.

Tournament organisers made the announcement on Tuesday, a day after Serena opened her clay-court season with a straight-sets win over Swedish qualifier Rebecca Peterson.

It's the third straight tournament from which 37-year-old Serena has withdrawn due to physical problems.

She retired due to a viral illness during her third-round match against Garbine Muguruza at Indian Wells and pulled out with a left-knee injury before her third-round match at the Miami Open in March.

As a result of this latest withdrawal, Venus advances into the third round via a walkover.

It is not immediately clear if Serena will recover in time to play at Roland Garros, the second major of the year, which starts in less than two weeks.