Mirjana Lucic-Baroni was upset by Jelena Ostapenko as two teenagers reached the Volvo Car Open final on Saturday.
Ostapenko, 19, edged past Croatian 11th seed Lucic-Baroni 6-3 5-7 6-4 in their semi-final in Charleston.
It continued a fine run for the Latvian, who also beat Caroline Wozniacki on the green clay at the WTA Premier event on her way to a third Tour final.
Ostapenko needed more than two hours to get past Lucic-Baroni, the 35-year-old playing her third semi-final of the year.
After dropping the second set, Ostapenko rolled out to a 4-1 lead in the decider and while she gave up one of her breaks, she closed out the win.
Awaiting her in the final is another 19-year-old in Daria Kasatkina, the Russian having overcome Laura Siegemund 3-6 6-2 6-1.
Kasatkina, the world number 42, reached her maiden WTA decider with victory in two hours, 18 minutes.
Ostapenko, who is 0-2 in finals, has won each of her previous meetings with Kasatkina, including at Eastbourne last year.