The 19-time Grand Slam champion will face Lucie Safarova on Saturday with a chance for a third title at Roland Garros.
Williams also didn't attend a pre-final news conference, instead issuing a statement in a question-and-answer format that was distributed by the tournament.
"I started to feel unwell around the third or fourth round," Williams' statement begins, "and I felt really terrible during the semis" on Thursday, when she came back to beat Timea Bacsinszky after dropping the first set. "I'm actually not sure how I got through the match, and when it was over, I just kind of collapsed. I couldn't move.
"I think I have some kind of flu, which makes it tough, because it's just a matter of resting and keeping hydrated -- there's not much else I can do. I've felt really cold so I'm just fighting that, trying to sweat it out."
Williams added that a doctor would be visiting at her apartment later Friday.
"We'll see if he can do anything else to help, but I don't think there's anything," Williams said. "... At this point, I just want to get better -- it's hard to think about the match or winning another Grand Slam title right now."
The 33-year-old American would be the second-oldest women's champion in the history of the French Open if she can beat Safarova, who will be participating in the first Grand Slam singles final of her career.
The Associated Press contributed to this report.