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.
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.
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.