Daria Kasatkina shocked Caroline Wozniacki to knock the Australian Open champion out of the St Petersburg Ladies' Trophy.
World number one Wozniacki, who won her first Grand Slam title in Melbourne last month, was ousted in straight sets as eighth seed Kasatkina claimed a 7-6 (7/2) 6-3 win on Friday.
Victory for the Russian world number 23 set up a semi-final clash with defending champion Kristina Mladenovic, who beat the Czech Republic's Katerina Siniakova 6-3 6-4.
Mladenovic, who won the Australian Open doubles title with Timea Babos last month, defeated Siniakova in 78 minutes to win her seventh straight game in St Petersburg.
"It was a very good match," Mladenovic said on court after the match. "I think she's a very good mover, and the first set was such high quality on the serve, from both of us. It was very difficult to have opportunities to break each other."
Earlier, Petra Kvitova upset world number six Jelena Ostapenko 6-0 6-2 in a dominant display.
The two-time Wimbledon champion took just 58 minutes to dismantle Ostapenko and broke serve five times.
"At the end of the last match, I found my rhythm a little bit - I knew I had to play a little bit better today, against someone who is playing very aggressive," Kvitova said on court after the match.
"I had to stay focused on every point, which I did.
"It was just about a few games which I was serving and kept my serve. I served pretty well today, so it was great."
She will play Julia Goerges who overcame 18-year-old Russian Elena Rybakina 6-3 6-3.
The in-form German, who sits at a career-high ranking of 12th in the world, has won 18 of her last 19 matches.
She is also looking for her fourth title in her last five tournaments after winning the Kremlin Cup and the WTA Elite Trophy last year and the ASB Classic last month.