Albert Ramos-Vinolas and Fabio Fognini set up a quarter-final meeting at the Kremlin Cup with three-set wins on Wednesday.
Ramos-Vinolas, the second seed at the ATP 250 event, battled past Evgeny Donskoy 6-4 6-7 (4-7) 6-4, while Fognini beat fellow Italian Paolo Lorenzi 7-5 4-6 6-1.
Also advancing to the last eight was Stephane Robert, the French veteran too good for Lukas Rosol in a 7-5 6-1 win.
In first-round matches, eighth seed Andrey Kuznetsov claimed a 6-3 5-7 6-2 win over Guido Pella and teenage qualifier Alexander Bublik beat Konstantin Kravchuk 1-6 6-3 6-3.
Damir Dzumhur fought back from 3-0 down in the third set to overcome Karen Khachanov 3-6 7-6 (7-5) 6-4.