The 23-time Grand Slam champion was set to take to the court to face arch rival Maria Sharapova on Philippe Chatrier today.
But the American was struggling with injury and she dramatically pulled out less than an hour before the match was due to start.
A press conference was immediately called where Williams revealed all on the problem that was forcing her to quit.
Williams will now have an MRI scan on Tuesday to discover the full extent of the problem with the possibility of her playing Wimbledon on a knife-edge.
An emotional Williams said: “I unfortunately have been having some issues with my pec, my pec muscle, and it has been getting worse to the point where right now I can’t actually serve. It’s kind of hard to play when I can’t physically serve.
“It didn’t start before I got here. The first time I felt it was against Goerges in my last match. That’s when I started to feel it. I was, like, it was really painful and I didn’t know what it was.
“In my doubles yesterday I tried a lot of different tapings, and I tried lots of different types of support to see how it would feel under match circumstance. It didn’t really get a lot better.
“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, you know. All for this moment.
“So it’s really difficult to be in this situation, but I always, for now in my life, I just always try to think positive and just think of the bigger picture and hopefully the next events and the rest of the year.”
There were signs all was not well during Serena’s doubles clash with sister Venus yesterday where service speeds were noticeably down.
The news has taken everyone around Roland Garros by surprise. Earlier Serena’s coach Patrick Mouratoglou had insisted the America was ‘close to being 100 per cent fit’.
There are now fears Williams, who was playing her first Grand Slam since giving birth daughter Olympia, could be set for an extended time on the sidelines.
She said: “I’m going to get an MRI tomorrow. I’m going to stay here and see some of the doctors here, see as many specialists as I can. And I won’t know that until I get those results.”
This is the first time Serena has every withdrawn from a Grand Slam tournament.
“It’s very difficult, because I love playing Maria,” she added. “It’s just a match I always get up for. You know, it’s just her game matches so well against mine.
“It is difficult, because like I keep saying, I have given up so much to be here. You know, there is times where I’m on the court and I’m practicing, and I look on the monitor and I see my daughter and she’s playing and I want to be there, but I know that these are the sacrifices you have to make to live out your dream.
“And I have made every sacrifice that I could. So it’s extremely disappointing. But also, 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.
“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.”
