Lukas Lacko of Slovakia beat Robin Haase of Holland 6-4 6-4 to book a slot in the Zagreb Indoor semis.
He will now play Marcos Baghdatis or Ivan Dodig for a place in the final.
In the other quarterfinal, Mikhail Youzhny of Russia fought hard to defeat Ivo Karlovic of Croatia in three sets 7-6 (10) 3-6 7-6 (6).
Michael Berrer of Germany is also through, having won against the Austrian Jurgen Melzer 5-7 6-4 6-4.
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