Local hope Ivo Karlovic stunned Bulgarian seventh seed Grigor Dimitrov in the opening round of the PBZ Zagreb Indoors on Tuesday.
Karlovic, the world number 130, fired 18 aces on his way to a 7-6 (7-3) 7-6 (7-3) victory in which neither player faced a break point on the hard-courts in Croatia.
Meanwhile, Slovakia's fifth seed Martin Klizan rallied to defeat Italian lucky loser Matteo Viola 4-6 7-6 (8-6) 6-1 in two hours and 31 minutes.
German qualifier Philipp Petzschner booked a second-round clash with Italian second seed Andreas Seppi after beating Croatian wildcard Mate Pavic 7-5 6-2 in 61 minutes.
Dutchman Igor Sijsling prevailed 6-4 6-2 over Croatia's Nikola Mektic, and Czech Lukas Rosol won 6-2 6-4 against local hope Filip Veger.
Grega Zemlja accounted for Serbia's Ilija Bozoljac 6-7 (5-7) 7-6 (7-4) 6-2, while fellow Slovenian Blaz Kavcic overcame countryman Aljaz Bedene 6-7 (5-7) 6-3 6-3.
India's Somdev Devvarman advanced to the second round after Germany's Michael Berrer retired due to fatigue when trailing 0-6 7-6 (8-6) 4-1.