PARIS — Ill and sluggish, Serena Williams overcame a big deficit to reach the French Open final by beating 23rd-seeded Timea Bacsinszky of Switzerland 4-6, 6-3, 6-0 on Thursday.

With the temperature nearing 85 degrees on the hottest day of the tournament, Williams walked ever so slowly to changeovers, where even lowering herself to sit down seemed to be difficult. During those breaks, she pressed ice towels against her forehead and neck and guzzled water.

Her coach, Patrick Mouratoglou, acknowledged during a mid-match TV interview that the top-ranked Williams has been sick recently.

Trailing by a set and a break in the second, Williams won the final 10 games.

One victory from her third French Open championship and 20th major title overall, Williams faces Lucie Safarova in Saturday’s final.