The Czech earned a 7-5, 4-6, 9-7 victory over Sakkari to set up a showdown with 29-year-old Russian Anastasia Pavlyuchenkova in the final at Roland Garros. Pavlyuchenkova also advanced to her first major final after defeating unseeded Tamara Zidansek 7-5, 6-3 earlier Thursday.
It marked only the second time in the professional era that there were four first-time Grand Slam women semifinalists at a major tournament, and the first time since the 1978 Australian Open.
Krejcikova, who is a two-time major doubles champion, is competing in singles play in the main draw of a Grand Slam tournament for just the fifth time in her career. She is attempting to become the first Czech woman to win in Paris since Hana Mandlikova in 1981.
The 25-year-old Krejcikova also is trying to be the first woman to win both in doubles and singles at Roland Garros since Mary Pierce in 2000. Krejcikova and her partner, Katerina Siniakova, will compete in the semifinals Friday.
The No. 31-seeded Pavlyuchenkova, meanwhile, has played in more majors before reaching a final (52) than any other woman.
Pavlyuchenkova previously was 0-6 in Grand Slam quarterfinals matches before finally overcoming that hurdle Tuesday. She edged No. 21 seed Elena Rybakina to reach the semis.
On the men's side, 13-time French Open champion Rafael Nadal and 2016 winner Novak Djokovic will battle Friday in the semifinals. The other semifinal matchup will be No. 5 Stefanos Tsitsipas versus No. 6 Alexander Zverev.