The 23-time Grand Slam champion pulled out of her fourth-round match against Maria Sharapova due to a pectoral injury.
"I unfortunately have been having some issues with my pec, my pec muscle, and (it) has unfortunately been getting worse to the point where right now I can't actually serve".
Williams said she would get an MRI on Tuesday and "see as many specialists as I can" before determining whether she can play Wimbledon in four weeks.
Saturday's head-scratching back-and-forth began with Simons asking, "I know you want to get back to (daughter) Olympia, work with me here, please". "If I didn't play my best tennis, I realised that it would be very hard". It hasn't been easy.
"I think there are a lot of things in her game that she's done much better than I have".
"I was looking forward to my match against Serena and am disappointed that she had to withdraw", Sharapova said.
"I think Serena can get close to winning another Grand Slam".
"I couldn't have imagined I'd have come back so soon after giving birth and I needed 9-10 months before I was ready".
Sharapova thumped sixth-seed Karolina Pliskova 6-2 6-1 in the afternoon, winning almost 60% of her return points versus last year's women's ace leader and Roland Garros semifinalist.
The clues were all there during her doubles loss on Sunday, with a reduced serve speed, a 6-0 third set loss and walking off the court without carrying her own bag. I think this is just one of her best surfaces, and she always does really, really well here.
"It's hard because I love playing Maria", Williams said.
Every Grand Slam event constitutes some kind of statistical landmark for Williams, and this was an improbable one, given what can befall any top player over a two-decade-plus career: the first time she'd ever withdrawn before a match in a major. "I've done a lot of fighting and this is just the beginning".
"She worked like I have never seen her work before", Mouratoglou said of Williams' recent training stint at his academy in the south of France. "I was trying what I could, but it just wasn't enough".