Barty then lost the initial 12 points of the second set to trail 3-0 -- and yet somehow won it.
So it was fitting, perhaps, that Barty not only fell behind by a break in the deciding set before coming back to take control, but also that she required a half-dozen match points to finally close things out.
Barty, an Australian seeded No. 8, reached her first Grand Slam final by steadying herself and emerging to beat Anisimova, an American ranked 51st, by a score of 6-7 (4), 6-3, 6-3 in a topsy-turvy contest on a windy, rainy Friday.
"I played some really good tennis. I played some pretty awful tennis," said Barty, a 23-year-old who took nearly two years away from the sport starting in 2014 to switch to cricket before returning to the tour.
"I'm just proud of myself the way I was able to fight and scrap and hang in there and find a way," she said, "when I kind of threw away that first set."
After ending Anisimova's breakthrough run, Barty now takes on another unseeded teen for the championship Saturday: 19-year-old Marketa Vondrousova of the Czech Republic.
Vondrousova, ranked just 38th, reached her first major final by overcoming a shaky start in each set and eliminating No. 26 Johanna Konta of Britain 7-5, 7-6 (2).
Vondrousova has not dropped a set in the tournament and can become the first teenager to win the French Open since Iva Majoli in 1997.
"Best week of my life so far," Vondrousova said. "I'm just very happy with everything."
That was Barty's mood at the outset of her semifinal, which then took quite a turn. Several, actually.
With Anisimova serving down 0-5, 15-40, Barty held two sets points. From there, Anisimova began playing the way she did in her quarterfinal upset of defending champion Simona Halep -- and Barty suddenly lost her way. Anisimova took six consecutive games. In the tiebreaker, more of the same: Barty went up 4-2, but Anisimova collected the last five points.
"Pretty tough to come to terms with," Barty said of her first-set collapse. "Probably never done that to myself before."
When Anisimova claimed that set with a forehand winner, she raised both arms aloft, looking like someone who had just won the entire match. Not quite, kid.
Still, that momentum carried her to a lead in the second set, before Barty went on a seven-game run of her own to go ahead 1-0 in the third.
Not ready to quit, Anisimova broke to lead 2-1 and had just fended off three break points when, at deuce, her coach signaled from the stands that play should stop because the rain picked up. Anisimova paused while the chair umpire climbed down to check whether the lines were slippery but determined play could continue.
The little pause might have been enough to break Anisimova's concentration.
Barty broke there and went on a four-game burst. As the match slipped away, Anisimova had distress written all over her face. After one lost point, she clutched her racket against her chest like a pillow. After another, she balled up her right hand into a fist and landed a punch on each thigh.
"She just outplayed me, basically," Anisimova said.
Vondrousova did not start her major semifinal debut well, either. She double-faulted twice in the opening game while ceding the first 10 points and faced three set points down 5-3.
On Konta's first chance, she badly missed a swinging forehand volley.
"That's what I would do nine times out of 10. And probably nine times out of 10, it probably would go in," Konta said. "I definitely don't regret anything."
Perhaps, but she never recovered in that set. And in the second, Konta again blew a 5-3 edge.
Konta, the only member of the women's final four with previous Grand Slam semifinal experience, is now 0-3 in that round at majors. This time, she was undone by Vondrousova, a left-hander who appears to possess every shot there is, with a variety of speeds and angles.
"She's a tricky player," Konta said. "That's what she does well."
The semifinals were played in difficult conditions, in drizzle, wind that reached 12 mph (20 kph) and temperatures of about 60 degrees (15 Celsius). The matches also were played at unusual courts -- scheduling that was criticized by women's tennis tour CEO Steve Simon as "unfair and inappropriate."
Normally, all French Open singles semifinals are held in Court Philippe Chatrier, with the women on Thursday and men on Friday. But after a full day of play was washed out by rain Wednesday, tournament officials shuffled the schedule. The women's semifinals were held simultaneously at the second- and third-biggest courts at Roland Garros instead of the main stadium, starting just after 11 a.m.
Asked whether this felt like a major semifinal, Konta replied: "I mean, in terms of the surrounding and the occasion, probably not. ... In terms of the match itself, probably."