Krejcikova advanced Wednesday by eliminating 17-year-old Coco Gauff 7-6 (6), 6-3.
This is only the second time in the professional era that there has been four first-time semifinalists at any major tournament, according to the WTA. It also happened at the 1978 Australian Open.
In the men's quarterfinals, 13-time champion Rafael Nadal's streak of sets won at Roland Garros ended at 36 but he quickly recovered to defeat Diego Schwartzman 6-3, 4-6, 6-4, 6-0. Nadal's semifinal foe will be Novak Djokovic or Matteo Berrettini.
Sakkari, who can become Greece's first Grand Slam singles champion, and Krejcikova, who is from the Czech Republic, are both 25. Each has won only one tour-level title. Neither had been past the fourth round at a major until now.
Sakkari lost her first seven third-round Slam matches, which raised some doubts that have since been erased.
"I thought about it a lot of times — that maybe that was my ceiling, and I could not get any higher in the rankings, playing better in tournaments," she said. "But this year I proved (to) myself that I'm actually playing really good."
Both Sakkari and Krejcikova dealt with early deficits Wednesday.
Swiatek, a 20-year-old from Poland who has looked untouchable on clay, jumped out to a 2-0 lead. But then Sakkari took over, winning eight of 10 games. When Sakkari smacked a backhand winner down the line to close a 15-stroke point that claimed the first set, she leaned over and punched the air with her right fist.
That ended Swiatek's set streak at Roland Garros, which dated to the beginning of last year's tournament, when she dropped only 28 games in all. She'd only lost 20 games this year through four matches.
But Sakkari used clean strokes — accumulating 26 winners, nine more than her opponent — and a strategy of serving to Swiatek's forehand to gain control.
"Obviously I know I can play better than today," Swiatek said. "Everybody has seen that."
Down 2-0 in the second set, Swiatek took a medical timeout and left the court with a trainer, returning with her upper right leg taped. During the break, Sakkari tried to stay warm by hopping and skipping side-to-side behind the baseline and did not lose a beat when play resumed.
Swiatek said her injury was not serious but did bother her before and during the match.
"I couldn't even sleep well yesterday. I slept, like, few hours," she said. "I think I was feeling everything twice as much as I should. It was hard to rationally just see what's going on."
In the day's first quarterfinal, Gauff led 3-0 at the outset, then 5-3, and held a total of five set points in the opener, but failed to convert any. Krejcikova grabbed that set by taking the last four points of the tiebreaker and reeled off 15 consecutive points during one stretch en route to a 5-0 edge in the second set.
Closing out the most important victory of her singles career was not easy, though: Krejcikova needed six match points to do it.
Krejcikova has won two Grand Slam doubles titles with Katerina Siniakova — and they're into the semifinals in Paris — but is playing in only her fifth major tournament in singles.
"Everybody, they just put a label on me like, 'Yeah, you play doubles. You are a doubles specialist.' But I never thought I just want to be a doubles specialist," Krejcikova said.
"So I was just working hard all the time. I just wanted to play singles. It was really, like, frustrating that I just wasn't able to get there," she said. "But I always felt ... sooner or later, I'm just going to get there."
Look at her now. She's ranked a career-high 33rd and on a 10-match winning streak in singles.
Krejcikova ended the nine-match run of the 24th-seeded Gauff, who is based in Florida and was the youngest French Open quarterfinalist since 2006.
Gauff's 41 unforced errors included seven double-faults — and after one, she mangled her racket frame by whacking it three times against the ground.
"My hitting partner told me this match will probably make me a champion in the future," Gauff said. "I really do believe that."
