The world number one looked distressed through large parts of her semi-final on a hot Court Philippe Chatrier, wrapping herself in ice towels between changeovers and walking wearily around the arena.
But remarkably she reeled off the last 10 games to reach Saturday’s final where she will face Lucie Safarova for a third Roland Garros title and 20th career major.
“I have never felt so sick,” 33-year-old Williams told US broadcaster ESPN.
“I caught the flu after my third round match and have been struggling ever since.
“I never thought I could win and I didn’t want to go to a third set.
“But I thought that if I was going to lose I had better go for winners. I was so tired and the next thing I knew I was in a third set. I don’t know what happened.”
On her prospects for Saturday’s final, she added: “I just hope I can get better from here.” -AFP