Defending champion Marin Cilic of Croatia was first through to the Zagreb Indoors quarterfinals by beating Dusan Lajovic of Serbia 6-4, 7-5 on Wednesday.
Fifth-seeded Cilic converted only three out of 10 break points, two of them Lajovic double-faults.
Cilic improved to a 16-1 record in his last five attempts here, winning titles in 2009, 2010 and 2013.
Earlier, Dudi Sela of Israel ousted sixth-seeded Lukas Rosol of the Czech Republic 6-3, 7-6 (2) in the first round.
Sela led by a set and 5-3 before Rosol broke back and forced a tiebreaker. But Sela moved into an early lead again and closed out the match.
Lucky loser Daniel Evans of Britain, who made the draw after the late withdrawal of Radek Stepanek with a heel strain, defeated another Czech, Jan Hajek, 6-3, 6-1.
Russian qualifier Andrey Kuznetsov beat 2008 champion Sergiy Stakhovsky of Ukraine 6-4, 7-6 (8) for his first tour win since July, while Andrey Golubev of Kazakhstan defeated Victor Hanescu from Romania, 7-6 (6), 4-6, 7-6 (5).