Second seed Alexandra Panova came from a set down to book her place in the semi-finals of the Baku Cup at the expense of Russian compatriot Nina Bratchikova.
Panova was broken twice in the opening set but came back strongly to secure a 2-6 7-6 (7/5) 6-4 victory over the sixth seed after almost three hours on court.
Mandy Minella, seeded three, saw her Baku challenge ended at the quarter-final stage by Russia's Olga Puchkova, who ran out a 2-6 6-1 6-2 winner.
Serbian fifth seed Bojana Jovanovski advanced to the last four with a battling 5-7 7-6 (7/3) 6-3 win over countrywoman Aleksandra Krunic.
American Julia Cohen completes the semi-final line-up after ousting Magdalena Rybarikova of Slovakia 3-6 6-3 6-4.