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Serena withdraws from Rome over injured knee

ROME -- Serena Williams has withdrawn from the Italian Open ahead of her second-round match against sister Venus because of an old left knee injury.

Tournament organizers made the announcement 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 the 37-year-old Serena has exited from because of physical problems.

She retired with a viral illness during her third-round match against Garbine Muguruza in Indian Wells, California, and withdrew 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 wasn't immediately clear whether Serena would recover in time to play the French Open, which starts in less than two weeks.