Serena Williams withdrew from the French Open on Monday, shortly before she was due on court to face Maria Sharapova in a much-anticipated fourth-round showdown.
The 36-year-old 23-times grand slam champion, making a comeback after maternity leave, told a news conference she had a pectoral muscle injury.
"Unfortunately I'm having some issues with my pec muscle. Right now I can't actually serve it's kind of hard to play," she said.
"I'm having an MRI (scan) tomorrow, I'm going to stay here to see the doctors and see as many specialists as I can. I won't know (how serious the injury is) until I get those results."
Williams had worked hard to get back in shape and was making her Grand Slam comeback at the French Open following the birth of her daughter Alexis Olympia last September.
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"Physically I'm doing great. It hasn't been easy. I sacrificed so much to be at this event. I'm going to continue to get better. I had such a wonderful performance in my first grand slam back," she added.
