Illness forced Petra Kvitova to withdraw at the St Petersburg Ladies Trophy, while defending champion Kiki Bertens reached the semi-finals.
World number eight Bertens defeated teenage qualifier Anastasia Potapova 6-4 7-6 (7-3) having staved off three set points in a second set in which she was 5-2 down.
Bertens will face Ekaterina Alexandrova next after she was handed a walkover when third seed Kvitova withdrew.
Top seed Belinda Bencic was ousted by Maria Sakkari in three sets, with Elena Rybakina beating Oceane Dodin in the other quarter-final.
At the Thailand Open, world number four Elina Svitolina suffered a surprise straight-sets loss to Nao Hibino, who was joined in the semi-finals by Magda Linette, Patricia Maria Tig and Leonie Kung.
— WTA (@WTA) February 14, 2020