'Unfortunately I'm having some issues with my pec muscle'
American 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-time Grand Slam champion, making a comeback after a maternity break, told a news conference she had suffered a pectoral muscle injury in her third round victory against Germany's Julia Goerges.
"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 told a packed press room.
"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.
Despite starting the tournament with a notional ranking of 451 and having played only four matches this year, she played impressively in her three wins in Paris, beating seeds Ashleigh Barty and Goerges in rounds two and three.
Her clash against twice French Open champion Sharapova, who she has beaten 18 times in a row, had received huge pre-match billing and the American said she was "beyond disappointed" that she could not take to court.
"I gave up so much to time with my daughter and my family and put everything on court for this moment," she said.
"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."
Sharapova, herself making a comeback after missing the last two French Opens following a doping ban, will face either Garbine Muguruza or Lesia Tsurenko in the last eight.
She was yet to comment on Williams' withdrawal.