South African slugger Kevin Anderson posted a first-round victory Wednesday at the U.S. Men's Clay Court Championship.

The fifth-seeded 6-foot-8 Anderson, a titlist in Florida last month, handled Brazilian Joao Souza 6-3, 6-4 in 1 hour, 54 minutes at River Oaks Country Club.

An opening-round upset came when American journeyman Bobby Reynolds upended eighth-seeded Croat Ivo Karlovic, 7-6 (7-5), 2-6, 6-3. The 2007 Houston champion Karlovic lost despite uncorking 19 aces on Day 3.

Top seeds Mardy Fish and John Isner will see their first action of the week here on Thursday, when the 2006 Houston champ Fish faces fellow American Michael Russell and the 6-foot-9 Isner takes on Argentine Horacio Zeballos.

The winner of this $442,500 event will take home $79,880.