HOUSTON (AFP) - Former winner Ivo Karlovic continued his domination over German Benjamin Becker, winning 7-5 (7/5), 6-4 in the first round of the US Men's Clay Court Championship on Tuesday. Qualifier Karlovic now has six victories and no losses against Becker. He also beat Becker en route to the Houston title four years ago. "It was humid in the beginning and I had to adjust a little bit but it was good," said Karlovic, who is on the mend from an Achilles' injury. Sixth seeded Kei Nishikori of Japan advanced by beating Croatian qualifier Franco Skugor 6-3, 6-2, and American Ryan Sweeting rolled over Tim Smyczek 6-2, 6-2. First seed Mardy Fish won't play his first match until Thursday. Fish is the top-ranked American male for the first time in his career after vaulting past Andy Roddick.