Local favourite Ivan Ljubicic was the only seed to reach the quarter-finals of the Zagreb indoor tournament after overcoming Dane Kenneth Carlsen 7-6, 6-4 on Thursday.
The top-seeded Croatian, who will face Russian Mikhail Youzhny the last eight, broke Carlsen in the third game of the second set to seal victory in 85 minutes.
Four seeds exited the tournament on Thursday.
In an all-Croatian showdown, Ivo Karlovic defeated fifth seed Mario Ancic 7-6, 7-6.
Serbian teenager Novak Djokovic continued his fine form with a confident 6-3, 6-4 win over fourth-seeded Czech Radek Stepanek.
In Friday's quarter-finals he will meet fellow Serb, qualifier Ilija Bozoljac, who overcame Spanish eighth seed Feliciano Lopez 7-6, 6-2.
Russia's Youzhny had to work hard to oust sixth seed Jarkko Nieminen of Finland 2-6, 7-6, 7-6.
Italy's Andreas Seppi, who beat Croatian teenager Marin Cilic 4-6, 6-4, 6-4, will play Britain's Tim Henman in the quarter-finals.