Delray Beach, FL (SportsNetwork.com) - Towering Croat Ivo Karlovic was a hard-fought second-round winner Wednesday at the Delray Beach Open tennis event.

The 6-foot-10 fourth seed Karlovic fired 21 aces before sneaking past 18-year- old Aussie Thanasi Kokkinakis in a pair of tiebreaks, 7-6 (7-4), 7-6 (7-2), on the hardcourts at Delray Beach Tennis Center.

"It was really difficult because he [Kokkinakis] can really hit those forehands, and also it was a little bit windy," said the world No. 29 Karlovic. "I was hanging in there. I was serving really well, and in the tiebreaks I was really good."

Karlovic, nicknamed "Dr. Ivo," was the Delray Beach runner-up back in 2010.

Also on Day 3, fifth-seeded Frenchman Adrian Mannarino mauled American wild card Denis Kudla 6-2, 6-2 to reach the quarterfinals.

"It was a good thing for me that the conditions were very cold and pretty slow because I was able to move very well on the court, and Denis was missing a little bit more than me," said Mannarino.

The 2015 Delray champion will earn $84,250.