Australian Open champion and world number one Caroline Wozniacki lost 7-6 (7/2), 6-3 to local player Daria Kasatkina in the quarter-finals of the St Petersburg Ladies Trophy on Friday.
Fresh from her first ever Grand Slam title the Dane had raced to the quarter-finals here but had been expecting the Russian to give her a difficult time.
"It`s gonna be difficult but I`m excited for it," the 27-year-old said ahead of he clash with the sixth seed Kasatkina.
Fourth seed and Defending champion Kristina Mladenovic of France made an important step to retaining her title with a straight-sets win over Czech Katerina Siniakova on Friday.
Mladenovis, ranked 10th in the world, prevailed 6-4, 6-3 to book a semi-final spot and to record her second win over the 21-year-old Siniakova in their fifth head-to-head meeting. Mladenovic will face Kasatkina.
"It was a very good match as she (Siniakova) has been on great all season," Mladeniovic said on-court after the match. "I`m very happy with the intensity that I`ve put into the game today.
"No doubts the semi-final will be extremely difficult as Dasha (Kasatkina) is such a talented youngster and she always plays at her top in front of her home crowd."
Meanwhile, Czech two-time former Wimbledon champion Petra Kvitova cruised comfortably into the last four losing just two games in her 58-minute match with reigning French Open champion Jelena Ostapenko of Latvia 6-0, 6-2.
In the match for a place in the final Kvitova, who appeared in the main draw courtesy of a wild card, will clash with Germany's Julia Goerges, who ousted another Russian Elena Rybakina 6-3, 6-3.
"At this tournament every opponent is very hard to beat and I will need to show my best tennis to win the semi-final," Goerges said.