18-year-old Tagger, who possesses a rare single-handed backhand, came back from a set down to stun second seed and two-time Grand Slam champion Krejcikova 6-7(5), 6-3, 7-6(4) in two hours and 55 minutes, ending her eight-match winning run in the process.
Krejcikova was coming off the back of winning the title in Athens last week - her first trophy since winning Wimbledon in 2024.
"An incredible match. To be honest, I remember Barbora Krejcikova from before; I saw her play at the Grand Slams and it was a great experience for me. So I was quite nervous before the match. But now I am really happy. I just beat her!" an ecstatic Tagger said after the win to reporters.
"She plays incredible tennis. She has a lot of titles, and her style? She is really original; she is different from other tennis players, so it is not easy to play against her.
"But honestly, it was a fantastic match. I really enjoyed it, even though it was maybe very close. I was nervous at times, but it was really amazing in that atmosphere," the Austrian teen added.
Tagger - who is the youngest player in the world's top 100 - is now into just the second WTA final of her career, having lost to Anna Blinkova at the Jiangxi Open in 2025.
Her opponent in Sunday's showpiece will be Ukraine's Snigur, after she claimed a fine 7-6(4), 6-4 win over 19-year-old Tereza Valentova.
Valentova led 5-3 in the first set but failed to serve out and also led by an early break in the second set. However, Snigur found a way past her opponent to book a spot in a maiden WTA final.
On facing Snigur, Tagger remarked: "Unfortunately, I don’t know Darja very well. Before the match, I watched her play for a while, but I think I will have to watch one of her matches before the final and look at her a little more closely."
Follow the Livesport Prague Open final here.
Hamburg Open
Anna Bondar reached the final of the Hamburg Open for a second consecutive year after winning a rollercoaster contest against qualifier Elina Avanesyan, triumphing 7-6(3), 4-6, 7-6(2) in three hours and 29 minutes - the fourth-longest tour-level match of the season.
Bondar saved four set points in the opening set before throwing away a 4-2 lead in the second as the match headed into a decider.
A nail-biting final set saw the Hungarian fend off three match points, as she eventually got over the line to earn a shot at redemption. Bondar lost in last year's final to Lois Boisson, in what was her one and only WTA final appearance prior to this weekend.
She will face Tamara Korpatsch, who was handed a place in the final on home turf after her opponent, Mayar Sherif, retired from the match with the German leading 6-1, 2-0.