PARIS — After a resurgent first week at the French Open, Serena Williams was stopped short on Monday — not by her would-be opponent, Maria Sharapova, but by an injury to her right pectoral muscle that forced her to withdraw shortly before their fourth-round match was set to begin.

“Right now I can’t actually serve,” said Williams, 36, as she announced her withdrawal in an overstuffed interview room at Roland Garros. “It’s kind of hard to play when I can’t physically serve.”

This is the latest setback in Williams’s comeback after giving birth to her daughter, Olympia, last September.

“I’m beyond disappointed,” she said. “You know, I gave up so much, from time with my daughter to time with my family. I put everything on the court, you know. All for this moment. So it’s really difficult to be in this situation, but I always, for now in my life, I just always try to think positive and just think of the bigger picture and hopefully the next events and the rest of the year.”