PARIS - At changeovers in her French Open semifinal, an ailing Serena Williams walked ever so slowly to the sideline, where even lowering herself to sit seemed difficult.

With the temperature nearing 85 degrees, she pressed white towels filled with ice against her forehead and neck and guzzled water.

Early on, her play was as poor as her health. She failed to chase balls she normally would. As telling as anything: On those occasions when she did win points, Williams mostly refrained from her familiar fist pumps and yells of "Come on!"

Never can count her out, though, no matter the circumstances. Down a set and a break Thursday, and clearly not herself, Williams summoned the resolve to reach the final by beating 23rd-seeded Timea Bacsinszky, 4-6, 6-3, 6-0.

After getting broken to fall behind 3-2 in the second set, Williams claimed the final 10 games. She had a 12-2 edge in winners in the final set.

"Stunning," said Williams' coach, Patrick Mouratoglou. "This is the difference between champions and everyone else. There is no logical explanation." Mouratoglou said the No. 1-ranked Williams has been dealing for several days with the flu, including a fever and difficulty breathing.

Williams skipped her news conference and issued a statement reading: "I have been feeling unwell for a few days, and . . . I needed to see the tournament doctor."

Now one victory from her third French Open championship and 20th major title in all, Williams faces 13th-seeded Lucie Safarova of the Czech Republic on Saturday.

"I tried everything. I thought if I lose, I will lose with a fight," Williams told the crowd in French. "I tried, I tried. I found the energy. I don't know where, but I found it. And I won. I hope that on Saturday, I hope . . . "

Cutting herself off, she stepped away from the microphone, bent over and began coughing. She offered a quick wave, collected her things and left. Off the court, she got a hug from Mouratoglou, who helped her down stairs toward the locker room.

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"I was worried," said Williams' mother, Oracene Price. "But I knew if she could get through the second set, somehow maybe the adrenaline and God would help her get through the match."

Next comes Williams' 24th Grand Slam final, and Safarova's first. The lefthanded Safarova eliminated defending champion Maria Sharapova in the fourth round and 2008 champion Ana Ivanovic, 7-5, 7-5, Thursday.

Williams won her semifinal despite dropping the first set for the fourth time in six matches. She'd never fashioned that many comebacks during one major tournament.

When the going gets toughest, no one is better than Williams at the moment -- and, perhaps, in the history of the game. In 2015, she is 31-1, including 11-0 in three-setters.