The 23-time grand slam singles champion had showed great promise in her first major since giving birth to daughter Alexis Olympia last September.
She was unable to say whether or not she would be fit for Wimbledon which gets underway in four weeks' time.
Serena Williams's first grand-slam tournament since giving birth previous year has come to an end for her.
"I gave up so much".
"It's really hard to be in this situation, but... for now in my life, I just always try to think positively and just think of the bigger picture and hopefully (I can play in) the next events and the rest of the year".
Serena Williams withdrew prior to her blockbuster match with Maria Sharapova, citing a pectoral muscle pull.
Yesterday's match was to have been the first between Williams and Sharapova in more than two years.
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.
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"It's very hard", Williams added.
The 36-year-old said she had never had this type of injury before. She said she tried to tape it up to add support, but "it didn't get a lot better".
She added: "I'm going to get an MRI today and stay in France and see the doctors here, as many specialists as I can".
Yet less than an hour before they were likely to be on court, Williams called an impromptu press conference to announce she would have to withdraw.
Williams said she was gutted to miss out on facing her rival.
"But as soon as I started playing, I thought "no, too soon". I sacrificed so much to be here'.
"I don't have any negative feelings towards her, which again, was a little disappointing to see in that hearsay book... as a fan, I wanted to read the book and I was really excited for it to come out and I was really happy for her".
Her withdrawal allows Sharapova, a two-time French Open champion, to move into the quarterfinals. "Her game matches up so well against mine".
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