Top-seeded Marin Cilic of Croatia ousted Blaz Kavcic 6-3, 7-6 (5) on Friday to set up a Zagreb Indoors semifinal against defending champion Mikhail Youzhny of Russia.
Earlier, fourth-seeded Jurgen Melzer of Austria earned the 300th tour win of his career and will play Robin Haase of the Netherlands in the other semifinal.
Cilic is a two-time winner in his home event but missed last year's tournament with an injury. He broke Kavcic twice in the opening set and then traded breaks with the Slovenian in the second before prevailing in the tiebreaker.
"It was really great until 5-0 in the first set. I felt great on court," Cilic said. "He served better in the second set and it caused me problems ... but not so much as to force the third set."
Kavcic said Cilic "did not make a single mistake during the first set."
"If he keeps on playing like that, there is no doubt who will win the title," Kavcic said.
Youzhny, seeded third, reached the semifinals of the event for the third time by defeating Croatia's Ivan Dodig 6-3, 6-2 after never facing a break point. Youzhny 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."
Melzer defeated Lukas Rosol of the Czech Republic 6-4, 6-4 in the quarterfinals. Haase ousted Philipp Petzschner of Germany 6-4, 3-6, 6-0.
Melzer jumped to a 5-1 lead in the first set and saved four of the five break points he faced in the second, winning the last three games to seal the victory.
Haase served 11 aces and broke Petzschner seven times while saving nine of the 12 break points.