In this photo provided by the French Tennis Federation (FFT) Serena Williams, of the U.S, answers journalists at the Roland Garros stadium in Paris, Monday, June 4, 2018.
Serena also revealed that she will stay in Paris to undergo an MRI on Tuesday.
Serena Williams tried to play through the pain of an injured chest muscle. It's never an easy match against a grand slam champion.
Their French Open fourth-round showdown was billed as a grudge match - the chance for Williams to hit back at Sharapova's comments about her in her autobiography and an opportunity for Sharapova to end an 18-match, 14-year losing streak against the American.
She first felt the ailment against third-round opponent Julia Goerges on Saturday but still played doubles with sister Venus on Sunday.
This was Williams' first major tournament in 16 months, and her first as a mother. Her baby was born last September.
But shortly before the two old warriors were due on court, the 23-times grand slam champion stepped into a news conference room to announce she was pulling out.
"It's a Wimbledon final, you know".
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"I think there is a lot of things in her game that she's done much better than I have". So it's extremely disappointing. "I didn't expect to be reading a book about me, that wasn't necessarily true".
"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 and try to get better and not get it to a point where it could be a lot worse. I wish her a speedy recovery and hope she returns to the tour soon".
Williams won the championship in 2002, 2013 and 2015, while Sharapova won the championship in 2012 and 2014 at the French Open.
Oddly enough, Muguruza didn't have to put forth much effort in the fourth round, either.
But she secured a far more straightforward passage into round four with a 6-2 6-1 victory in just 59 minutes.
Second seed Caroline Wozniacki was knocked out by Russian youngster Daria Kasatkina, who now meets American 10th seed Sloane Stephens. Big mistake. You don't cross Serena Williams that way!
She didn't tell him she was hurting? "I'm just going to do what the doctor thinks I should do".
"I made a promise to myself and to my coach and to my team that if I'm not at least 60% or 50%, then I probably shouldn't play".