Top seed Belinda Bencic crashed out of the Luxembourg Open with a straight-sets defeat to Liudmila Samsonova.
Bencic, the Olympic champion, had designs on winning her first WTA title of 2021 having lost in the final in Adelaide and Berlin during the calendar year.
However, there was more frustration for the Swiss, beaten in the quarter-finals of the US Open by champion Emma Raducanu, as she was swept aside 6-1 6-4 by Samsonova.
It is the second-best win of Samsonova's career by ranking, the Russian having defeated Kiki Bertens, then world number 11, back in March in Miami.
She will face Jelena Ostapenko for a place in the final after the former French Open champion saw off Alize Cornet 7-6 (8-6) 6-2.
Marketa Vondrousova twice trailed by a break in the first set against Elise Mertens but battled back to see off the second seed 7-5 6-2. She now faces a semi-final with Clara Tauson, the teenager having breezed past Maria Bouzkova 6-3 6-2 as she seeks her third title of the year.
At the Zavarovalnica Sava Portoroz, Kaja Juvan will face Alison Riske after the Slovenian won two matches on Friday. Jasmine Paolini will meet Yulia Putintseva in the other last-four clash.