"I'm having an MRI (scan) tomorrow, I'm going to stay here to see the doctors and see as many specialists as I can".
The 36-year-old said she couldn't "physically serve" due to the pectoral problem, adding she first felt the injury in the third round against Julia Goerges on Saturday and then again in Sunday's doubles tussle with older sister Venus.
"It's very hard because I love playing Maria", Williams said.
Sharapova, seeded 28th at her first French Open since 2015, will play No. 3 Garbine Muguruza or Ukraine's Lesia Tsurenko in the quarterfinals.
"I unfortunately [have] been having some issues with my pec muscle", she said at a press conference.
"In my doubles I tried a lot of different tapings and different types of support to see how it would feel under match circumstance, and it didn't get a lot better".
This is the first time-in over two decades of Grand Slam play-that Serena Williams has withdrawn in the middle of a Grand Slam tournament.
But because no match-up in a women's field packed with potential Grand Slam winners provides the "edge" that is so apparent whenever the two spiky old warriors face each other across a tennis net.
"If you add all the things that we're able to do in our lives, and the amounts of people that we're able to inspire from our stories, from our background, where we came from, kind of what we have been able to do outside the sport, in our own ways, I think (it's) very inspiring".
Inside Tennis' Bill Simons asked Williams about Trump's alleged comments from 2004 that Sharapova's "shoulders were incredibly alluring" and that Williams was "intimidated by her supermodel good looks".
She was also playing just her third tournament of the year after giving birth to daughter Olympia in September. I can't say I have been intimated by anyone. I've made every sacrifice I could. Its extremely disappointing but I made a promise to myself that if I'm not at least 50 per cent, I shouldn't play.
Williams: "I honestly don't have any thoughts about that".
Moving on to their latest matchup, the record books show that Williams has dominated Sharapova, holding a 19-2 record with only four matches going to three sets.
"It's been getting worse to a point where right now I can't actually serve".
Halep has now reached eight quarter-finals in the 10 tournaments she's played this year as the Romanian continues to chase her maiden Grand Slam title.
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