Fifth-seeded Martin Klizan of Slovakia came back from a set and a break down to beat Italian lucky-loser Matteo Viola 4-6, 7-6 (6), 6-1 Tuesday in the first round of the Zagreb Indoors.
After winning the first set, Viola led 3-2 in the second when Klizan broke back. Klizan later wasted two set points before winning the set in a tiebreak.
Croatia's Ivo Karlovic served 18 aces to upset seventh-seeded Bulgarian Grigor Dimitrov in straight sets and two tie breaks, 7-6 (3), 7-6 (3). Dimitrov, who played his first career ATP singles final in Brisbane in January, did not concede a break but could not find the game to counter the strong-serving, towering Croat.
"This match was tough," Dimitrov said. "I have practiced with Ivo quite a few times and I knew what to expect. "
Karlovic, who had not previously managed a single victory this season, said "I finally won."
He added: "Home crowd helped... I always feel good here."
Grega Zemlja from Slovenia rallied from one set down to beat Ilija Bozoljac from Serbia 6-7 (5), 7-6 (4), 6-2. Somdev Devvarman of India also came back from a set and a break down to defeat two-time Zagreb finalist Michael Berrer, who retired because of fatigue when trailing 0-6, 7-6 (6), 4-1.
Also, Igor Sijsling of the Netherlands defeated Nikola Mektic of Croatia 6-4, 6-2, Philipp Petzschner of Germany beat Mate Pavic of Croatia 7-5, 6-2, and Lukas Rosol of the Czech Republic beat another Croat, Filip Veger, 6-2, 6-4.