The American looked completely out of sorts during her three-set semi-final win over Swiss outsider Timea Bacsinszky and said she "collapsed" once she walked off court on Thursday.
"I started to feel unwell around the third or fourth round and I felt really terrible during the semis against Timea," said Williams, who has been battling a bout of flu.
"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."
Williams, who will be chasing a 20th singles grand slam title on Saturday, is favourite to win her third French Open crown but knows her fitness could be an issue.
"I saw the tournament doctor on site and since I came home I have been resting, I just could not practise today," added the 33-year-old, who coughed and spluttered throughout the semi-final.
"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 felt really cold so I am just fighting that, trying to sweat it out. A doctor is coming to see me... and we'll see if he can help... but I don't think there's anything.
"It's just time. I need time and obviously don't have a lot of it."
Looking ahead to meeting the Czech 13th seed, who has reached the final without dropping a set while Williams has survived four three-setters, the world number one added: "Lucie has been in great form here.
"I know I'm going to have to play really well to win but, at this point, I just want to get better.
"It's hard to think about the match or winning a grand slam title right now. It's very upsetting that I feel so lousy right now."
(Editing by Ken Ferris)
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