Ivo Karlovic remained unbeaten against Benjamin Becker, winning 7-5 (5), 6-4 in the first round of the U.S. Men's Clay Court Championship on Tuesday.
Karlovic, a Croatian qualifier, improved to 6-0 in his career against Becker. He also beat the German on the way to the 2007 U.S. Men's Clay Court title.
Karlovic is recovering from an Achilles' injury that caused him to drop in the ATP rankings.
No. 5 seed Becker and Karlovic were making their first appearance in Houston since 2007.
''It was humid in the beginning and I had to adjust a little bit but it was good,'' Karlovic said.
No. 6 seed Kei Nishikori of Japan and American Ryan Sweeting also advanced. Nishikori beat Croatian qualifier Franco Skugor 6-3, 6-2, and Sweeting cruised past countryman Tim Smyczek 6-2, 6-2.
Sweeting will face another American in the second round, Sam Querrey, who was the runner-up in the 2010 tournament.
Top-seeded Mardy Fish of the United States won't play his first match until Thursday. Fish is the top-ranked American male for the first time in his career. He surpassed good friend Andy Roddick for the top spot.
''I'd be lying to you if I didn't say it has a nice ring to it,'' Fish said.
''I've put a lot of work in on the court the past two years.''
