"It's very hard because I love playing Maria", Williams added after being denied a chance to add to her three French Open crowns and possibly equal Margaret Court's all-time record 24 grand slam singles titles in Paris.
"I've unfortunately been having issues with my pec muscle and unfortunately, it's been getting worse", Williams told reporters in a press conference.
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.
She was unable to say whether she would be fit for Wimbledon which gets underway in four weeks. U.S. tennis player Serena Williams has pulled out of her fourth-round match with Russia's Maria Sharapova who will now progress to the quarter-finals of the French Open tournament.
"I'm beyond disappointed. You know, I gave up so much, from time with my daughter, to time with my family".More news: Kanye West officially releases new album Ye
The shock withdrawal, Williams' first in her 20-year career that she had pulled out during a Grand Slam event, came just minutes before she was due on Court Philippe Chatrier to face Sharapova. The most recent match was in the 2014 Canadian Masters, a match Sharapova won in three sets.
Halep had earlier thumped Elise Mertens and said of facing the German: 'Well, it's every time it's a tough match, and every time I expect the same. "It's a match I always get up for and her game matches up against mine". She told reporters, "I thought it was a flawless opportunity to see how I was going". It also was an opportunity to test things out to see if there was a way she could manage the injury, in anticipation of the singles Monday.
Sharapova turned the situation around, but Garcia got a lot of praise.
The match against Sharapova had extra intrigue because the two players had not faced each other since Sharapova served a 15-month doping ban.More news: China criticizes 'protectionism' after U.S. steel tariff hike
Sharapova and Williams have met 21 times, with the American leading the head-to-head record 19-2 after 18 consecutive wins. That's when I started to feel it.
"I'm beyond disappointed", she said. I sacrificed so much to be here'.
"And I was one of the few people that, [with] her whole drug incident, I was, like, "She was fearless to say something".
Maria Sharapova and Garbine Muguruza meet in the quarterfinals of the 2018 tennis French Open.More news: No apology to Monica Lewinsky, says Bill Clinton
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