Elise Mertens dumped reigning champion Cagla Buyukakcay out of the Istanbul Cup on Friday with a 6-4 4-6 6-2 success.
Buyukakcay had overcome Danka Kovinic in the final of this WTA International event 12 months ago, but there will be no repeat triumph as she fell to the Belgian sixth seed.
Mertens broke her opponent three times in the decider to set up a semi-final meeting with third seed Irina-Camelia Begu, who came from a set down to defeat Basak Eraydin 2-6 7-6 (7-5) 6-1.
The other last-four clash sees Jana Cepelova go up against top seed Elina Svitolina, who came through 6-4 6-4 against Sorana Cirstea.
Earlier, Cepelova fought back to oust Dayana Yastremska 2-6 7-6 (7-4) 6-4.