Ivo Karlovic was an early casualty at the US Men’s Clay Court Championship after his straight-set loss to Jack Sock.
The eighth seed never got going against the home hope, with Karlovic broken four times en route to a 6-1 6-4 defeat.
It turned out to be a good day for the Americans in Houston as Sam Querrey and Robby Ginepri both registering first-round wins.
Querrey had just enough to see off Alex Bogomolov Jr 6-4 7-6 (8-6), while Ginepri held his nerve to win the crucial points in his 7-5 7-6 (8-6) win over Thanasi Kokkinakis.
Dustin Brown, though, spoilt the US party as he advanced with an efficient 6-1 6-2 win over wild card Rhyne Williams.
Marcos Baghdatis failed to build on a good start as the Cypriot lost 4-6 6-2 6-1 to Alejandro Gonzalez.