"It's extremely disappointing," Williams said during a news conference at Roland Garros. "But also, I made a promise to myself and to my coach and to my team that if I'm not at least 60 percent or 50 percent, then I probably shouldn't play."

Williams, who has won 23 Grand Slam singles titles, said she can't serve because of a problem with her right pectoral muscle. Her voice trembled a bit as she explained that she will get an MRI and consult with doctors before figuring out what she will do next. Her coach, Patrick Mouratoglou, said in an interview that an ultrasound revealed the injury to the muscle, but the extent wasn't clear.

"The fact that I physically can't serve at all is a good indication that maybe I should just go back to the drawing board and stay positive," Williams said, "and try to get better and not get it to a point where it could be a lot worse."

Williams, 36, was competing at a major tournament for the first time in 16 months -- and for the first time since giving birth to her daughter last September.

Williams said her chest began hurting in her third-round singles victory over 11th-seeded Julia Goerges on Saturday.

"It was really painful," Williams said, "and I didn't know what it was."

Despite that, Williams played in a doubles loss alongside her sister Venus on Sunday and tried to limit the pain by taping up the muscle. But nothing was effective enough that Williams thought she could continue in Paris.

Her withdrawal allows Sharapova, a two-time French Open champion, to move into the quarterfinals.

Their match had shaped up as the most-anticipated of the women's tournament.

In a statement released by the tournament, Sharapova wished Williams "a speedy recovery." This would have been their 22nd career meeting; Williams has won 19, including the past 18. Because of the withdrawal, this does not count as a victory for Sharapova, who will play 2016 French Open champion Garbine Muguruza on Wednesday.

Oddly enough, Muguruza didn't have to put forth much effort in the fourth round, either. Her opponent, Lesia Tsurenko, stopped because of an injury after only two games.

No. 1 Simona Halep, a two-time runner-up at Roland Garros, will play former No. 1 Angelique Kerber, a two-time major champion elsewhere, in Wednesday's other quarterfinal after both picked up easy straight-sets wins in the fourth round.

The quarterfinals Tuesday are Sloane Stephens vs. Daria Kasatkina, and Madison Keys vs. Yulia Putintseva. Kasatkina eliminated Australian Open champion Caroline Wozniacki 7-6 (5), 6-3 in a match that resumed Monday after being suspended a night earlier because of darkness at 3-all in the second set.

But the day's biggest buzz was about a match that never took place.

Williams said she first was troubled by her pectoral muscle during the best performance of her return, the 6-3, 6-4 win over Goerges. But Mouratoglou had no idea there was an issue.

She didn't tell him she was hurting?

"No, because she knew I would tell her not to play the doubles," he told the AP. "I would have made her cancel the doubles, believe me."

During Sunday's doubles, though, Williams showed signs of trouble. By the third set, she hit first serves at about 80 mph, about 40 mph slower than she can normally. Between points, she repeatedly jabbed a thumb into her chest to massage the muscle.

"At the end, she was just pushing the ball. ... That's when I knew she had a problem. I didn't know what kind of problem, but I knew there was something really wrong," Mouratoglou said. "Basically, she couldn't play."

Williams tried taping up the muscle for doubles, but that didn't help. And while she's "pretty much had every injury in the book," this was the first time dealing with this particular malady.

Mouratoglou said they held a practice session Monday morning, holding out hope that rain in the forecast would postpone the match against Sharapova.

"We might gain one more day, and you never know how it feels tomorrow. So we had to wait," he said. "But we saw the sky was still blue ... and we thought, 'No chance.'"

Sharapova's coach, Thomas Hogstedt, said that word of Williams' withdrawal arrived while his player was doing warm-up exercises outside the locker room at Court Philippe Chatrier, waiting for the match ahead of theirs to finish.

"She was keen on playing Serena. It's a little bit sad, I think, for everyone. Everybody looked forward to this match," Hogstedt said. "This match has had so much buildup."