John Isner and Fernando Verdasco highlighted Tuesday's first-round winners during Day Two of the $455,000 U.S. Men's Clay Court Championship.
The fifth-seeded Isner, the runner-up to gritty Argentine Juan Monaco in this event last year, prevailed in a first-set tiebreaker en route to a 7-6 (7-4), 6-4 decision over fellow American Ryan Harrison at River Oaks Country Club.
Verdasco also had to withstand an opening-set challenge in his match with Steve Johnson before taking down the American wild card by a 7-6 (7-5), 6-2 score.
Johnson was a two-time NCAA champion at the University of Southern California.
Isner's next opponent will be Jack Sock after the wild-card entry stopped qualifier and fellow American Bradley Klahn 7-6 (7-3), 6-3
In other action involving seeds, No. 7 Paolo Lorenzi of Italy subdued Austrian qualifier Gerald Melzer 6-2, 6-2 and oft-injured Robby Ginepri took out No. 8 fellow American Michael Russell 6-2, 6-2.
Meanwhile, Italian Flavio Cipolla overcame Argentine qualifier Facundo Arguello 4-6, 6-4, 7-5 and Lithuanian Ricardas Berankis outlasted Canadian Jesse Levine 6-7 (3-7), 7-5, 6-4.
This week's top seeds are Spanish star Nicolas Almagro, resurgent German Tommy Haas and the reigning champion Monaco. The former world No. 2 Haas captured this event in 2004 and will meet Berankis in his opener this week.
The 2013 Houston champion will collect $82,300.