Serena Williams battled illness and fierce heat to reach a third French Open final on Friday, defeating Timea Bacsinszky to move one win away from a 20th Grand Slam title.
The world number one and Paris champion in 2002 and 2013 was clearly ill and distressed for large parts of her semi-final.
She trailed by a set and a break in the second before racing to a 4-6, 6-3, 6-0 win, reeling off the last 10 games of the semi-final.
The 33-year-old American goes on to face Czech 13th seed Lucie Safarova, who beat Ana Ivanovic 7-5, 7-5 to reach Saturday's final, where victory will put her just two Grand Slam titles behind Steffi Graf's Open record of 22.
Friday's result was tough on Swiss 23rd seed Bacsinszky, who was making a living washing dishes and working in hotels just two years ago during an injury-enforced absence from the tour.
"I have never felt so sick," 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."
Williams who has now reached a 24th Major final, having come back from a first-set deficit four times at the tournament.
Williams was all business as usual in her first two service games, giving up just a single point. But Bacsinszky wasn't intimidated and broke for 3-2 with a searing crosscourt backhand off a Williams' second serve which could have been lifted from the American's playbook.
Williams, who went into the tie having lost just three of her 26 Grand Slam semi-finals, was clearly suffering in the 29-degree heat. Her breathing was laboured, she wearily demanded more and more ice-towels during changeovers, and was looking increasingly disorientated.
The blonde Swiss outsider had nothing to lose and played like it.
Having started a college course in hotel management during her absence from the tour, Bacsinszky was revelling in her moment in the sun, as fierce as it was on Court Philippe Chatrier.
She saw two set points disappear and then faced down a break point in the 10th game but wrapped up the opener when Williams overhit an easy forehand drive.
Bacsinszky's 16 winners were twice as many as her opponent, perfectly illustrating the requirement that attack is the only option against the American superstar whatever her state of mind.
Williams fought off five break points in the fifth game of the second set but was broken when she fired a desperate backhand wide.
At the changeover, Williams's coughing spurts became so loud that they echoed around the arena courtesy of the courtside microphones. But somehow she dug deep, retrieved the break, and then raced away with the second set by winning four straight games.
Rejuvenated, the top seed broke for 1-0 in the decider, backed it up with a double break for 3-0, and again for 5-0 as Bacsinszky wilted.
The contest was over when the Swiss fired a loose backhand, having won just 12 points in the entire final set.