A pectoral injury forced Serena Williams to pull the plug on Roland Garros before her fourth-round showdown with Maria Sharapova.

The 23-time Grand Slam champion underwent an ultrasound test today and is planning to undergo and MRI tomorrow to assess the state of her injury.

More: Serena Withdraws From Roland Garros

Williams' coach, Patrick Mouratoglou, said he's encouraged by her performance because she was only at about 80 percent of her level yet still reached the fourth round in just her third tournament in the last 17 months.

"I can't say she was 100 percent because she's been fitter than she was here," Mouratoglou told Tennis Channel's Jon Wertheim. "But coming from where she's coming from it's unbelievable that she's able to play like that, I think, first.

"And second, she was getting there. I think with two or three more weeks she would have been 100 percent."

Mouratoglou is convinced Williams' comeback after a 15-month maternity leave will succeed because she's more motivated than he's ever seen her.

"To be completely honest I was doubting it. Now I don't doubt at all that her motivation level is at the maximum I've seen," Mouratoglou said. "Maybe it sounds strange, but for her to come back is an incredible effort. She's a mom she wants to spend time with her family...

"But she has to devote a lot of time and change her life for tennis like travel around the world and if she does it, it's because she definitely wants it more than anything. If you had seen how hard she works the last five weeks, you wouldn't have any doubts about her motivation. She's worked more than I've seen her work ever."

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