The green clay tournament begins this weekend and will be played without three of the world's top 30 women's tennis players. Top-ranked Williams, 2009 tourney champion Sabine Lisicki, and three-time Grand Slam champion Maria Sharapova also pulled out Thursday.
Williams said in a statement released by the tournament that her knee "has not fully healed" and that she looks forward to "returning to the WTA Tour in Europe."
Story continues below ↓advertisement | your ad hereLisicki is out because of a left ankle injury. Sharapova announced her withdrawl due to her continuing recovery from an elbow injury.
The clay-court season picks up in earnest later this month in Europe, leading to the French Open, which begins May 23.
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