Maria Sakkari upset top-seeded Belinda Bencic to claim a spot in the St. Petersburg Ladies Trophy semi-finals on Friday (Saturday AEDT), while Petra Kvitova withdrew because of illness.
Bencic was a set and a break up before Sakkari stormed back to win 2-6 6-4 6-3.
"I'm really proud of the way I'm fighting and the way I can turn these matches around," the Greek star said.
That sets up a semi-final against Kazakhstan's Elena Rybakina, who beat Oceane Dodin 6-7 7-5 6-2 in their quarter-final.
The other semi-final pits second-seeded Kiki Bertens against Ekaterina Alexandrova, who advanced by walkover after Kvitova's withdrawal.
Kvitova said the unspecified illness bothered her on Thursday during a three-set win over Alison van Uytvanck.
"I wasn't feeling well during my match yesterday and hoped I would feel better today, but unfortunately, that's not the case," Kvitova said.
Bertens, who won the St. Petersburg title last year, needed nearly two hours to win her quarter-final 6-4 7-6 against qualifier Anastasia Potapova.