Transylvania Open favourite Anhelina Kalinina squandered a lead as she went down 6-7 (9-7) 6-1 6-3 to Anna Blinkova in the quarter-finals.
Kalinina, the second seed, looked well placed to go on and reach the last four in Cluj when she battled into the lead against Blinkova on Friday.
But the Ukrainian could not stop the world number 138 rallying back to reach her second WTA Tour-level semi-final of the season.
Blinkova went on to reach the final on her previous last-four appearance at a WTA tournament this year – at L’Open 35 de Saint Malo – but missed out on the title when she lost to Beatriz Haddad Maia.
Kalinina’s exit leaves Anastasia Potapova as the highest seed left in the tournament, and she will face fellow Russian Blinkova for a place in the final.
Potapova defeated Hungary’s Anna Bondar 7-5 3-6 6-1 to reach the last four.
Jasmine Paolini beat Jule Niemeier 7-5 7-5 to tee up a meeting with Nuria Parrizas Diaz or Xiyu Wang.