Top seed Marin Cilic overcame Blaz Kavcic 6-3 7-6 to reach the semi-finals of the Zagreb Indoors event.
Croatian Cilic, a two-time winner of the tournament on home soil, broke his opponent twice in the opening set and then traded breaks before sealing victory 7-5 in the tie-break.
Cilic will take on defending champion Mikhail Youzhny of Russia in the last four after he defeated Croatia's Ivan Dodig 6-3 6-2.
The number three seed broke for a 4-2 lead in the first set and then raced ahead 4-0 in the second.
"It was a good match for me, I played well and I like atmosphere here," Youzhny said. "I can't say this is the best surface, but when I am in Moscow at home I practice indoors a lot, and I am used to the conditions like this."
The second semi-final will see Jurgen Melzer of Austria take on Dutchman Robin Haase.
Melzer claimed a 300th victory of his career with a 6-4 6-4 victory over Lukas Rosol, while Haase ousted Philipp Petzschner of Germany 6-4 3-6 6-0.