Marketa Vondrousova © Gallo Images
World number 233 Marketa Vondrousova stunned Czech compatriot and top seed Barbora Strycova 7-6 (7/3), 6-2 to reach the Biel/Bienne final in Switzerland on Saturday.
The 17-year-old, who came through the qualifying competition at the Swiss hard court event, will face Estonia's Anett Kontaveit in Sunday's title match.
Kontaveit made the final with a 6-4, 4-6, 7-5 win over Aliaksandra Sasnovich of Belarus.
"I didn't know I could play this good," said Vondrousova after seeing off 31-year-old Strycova, the world number 18.