PARIS -- American tennis great Serena Williams pulled out of the 2018 French Open on Monday due to injury, sending her fourth-round opponent Maria Sharapova into the quarterfinals.

"Unfortunately I've been having some issues with my pec muscle. Right now I cannot actually serve, so it's hard to play when I can't physically serve," Williams told a press conference, just a moment before her fourth-round clash with Sharapova, which would have been their 22nd encounter.

The American revealed that the first time she felt the injury was against Germany's Julia Goerges last match.

"That's when I started to feel it. It was really painful and I didn't know what it was."

"In my doubles yesterday I tried a lot of different tapings, and lots of different types of support to see how it would feel under match circumstances. It didn't really get a lot better," she pointed out.

Williams says she never had a similar injury before, and really didn't know how to manage it.

"Sadly, when you do have an injury that you have had before, you can kind of manage it. I have pretty much had every injury in the book. But this is a little different, and I'm clueless as to what to do," she said.

Williams had won 18 straight matches against Sharapova prior to Monday's scheduled head-to-head.

"It's very difficult, because I love playing Maria. You know, it's just a match I always get up for and it's just her game matches so well against mine."

Williams also says she is upset about having to exit.

"I'm beyond disappointed. I gave up so much, from time with my daughter to time with my family. I put everything on the court for this moment.

"So it's really difficult to be in this situation, but I always try to think positive and just think of the bigger picture and hopefully the next events and the rest of the year," said Williams.

36-year-old Williams was playing her first Grand Slam tournament since giving birth to a daughter last September.

"I have been doing so good. Every match has been getting better for me. Physically I'm doing great.

"I can only take solace in the fact I'm going to continue to get better. I had such a wonderful performance in my first Grand Slam back. I just feel like it's only going to do better. Hopefully I can continue to heal and be able to play those events," added the 23-time Grand Slam singles title winner.

Williams is scheduled to get an MRI on Tuesday, and is going to stay and see doctors in Paris.

Williams says she will wait for those results before making any decisions about future training plans and Wimbledon.

"I made a promise to myself, my coach and my team that if I'm not at least 60% or 50%, then I probably shouldn't play."

"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, stay positive and try to get better, not getting it to a point where it could be a lot worse," she said.