Defending champion Mikhail Youzhny beat Igor Sijsling of the Netherlands 6-3, 6-2 in the second round on Wednesday to become the first quarterfinalist at the Zagreb Indoors.
The third-seeded Russian conceded only three points on his first serve as his record at the tournament improved to 10-4.
Earlier, in first round matches, sixth-seeded Marcos Baghdatis from Cyprus rallied to defeat Germany's Tobias Kamke 6-7 (5), 6-2, 6-4.
Kamke converted his second set point in the tiebreaker to take the lead, but could not keep up the pressure. Baghdatis won all 29 points on his first serve in the second and third sets.
"I did not expect him to play so good, I was a bit surprised," said Baghdatis, who was the 2007 champion here. "He was very aggressive ... I needed time to adjust."
Robin Haase of the Netherlands also came back from one set down to defeat Croatian wild card Antonio Veic 2-6, 6-3, 6-3. Flavio Cipolla made it to the second round after Lukas Lacko of Slovakia - who reached the final in Zagreb last year - retired at 1-1 in the first set because of a back injury.
In a late match, Croatia's Ivan Dodig beat Benjamin Becker of Germany 6-2, 7-6 (3).