Lukas Lacko of Slovakia ousted second-seeded Alex Bogomolov Jr. of Russia 6-0, 6-7 (1), 6-1 to advance to the Zagreb Indoors quarterfinals on Thursday.
Bogomolov, a quarterfinalist last year, rallied after dropping the first set but lost five games in only 14 minutes in the decider.
Third-seeded Mikhail Youzhny of Russia defeated Grega Zemlja of Slovenia 6-1, 6-3 to reach his third quarterfinal in Zagreb.
Defending champion Ivan Dodig beat Guillermo Garcia-Lopez of Spain 4-6, 6-4, 6-4, and said, ''It seems that I cannot win an easy match in Zagreb.
''This one was tough and the one tomorrow will most likely be even tougher.''
The fourth-seeded Dodig will face No. 6 Marcos Baghdatis, the 2007 champion, who downed Lukasz Kubot of Poland 6-4, 6-2.
''I kept the momentum,'' Baghdatis said.
Another Croat, eighth-seeded Ivo Karlovic won his second-rounder by hitting a dozen aces past Dudi Sela in beating the Israeli 6-3, 6-2.
Karlovic will play Youzhny on Friday.