Margarita Gasparyan and Daria Kasatkina have both won in the round of 16 as a record seven Russian players made the quarter-finals of the St Petersburg Ladies Trophy.
The eighth-seeded Kasatkina beat Aliaksandra Sasnovich of Belarus 5-7 6-3 7-6 (7-2) while Gasparyan ousted Katerina Siniakova of the Czech Republic 6-4 6-4 to give Russia a clean sweep of all four matches played Thursday.
Three other Russians advanced on Wednesday, giving the country seven players in the quarter-finals of a tournament for the first time.
The WTA Tour said the last time seven players from one country reached the last eight of a tour-level event was in 1993, when seven Americans were in the Oakland quarter-finals.
The other two matches on Thursday were all-Russian affairs.
Anastasia Gasanova rallied to beat fifth-seeded Anastasia Pavlyuchenkova 1-6 7-6 (10-8) 7-6 (7-4) despite being ranked 196 places lower at No.241.
Second-seeded Veronika Kudermetova needed nearly three hours to overcome Kamilla Rakhimova 7-5 3-6 7-6 (7-5).
Top-seeded Ekaterina Alexandrova, Vera Zvonareva and Svetlana Kuznetsova advanced on Wednesday. The sole non-Russian player in the last eight is Romania’s Jaqueline Cristian.