Milos Raonic and Juan Martin del Potro were among the first-round winners at the Delray Beach Open on Tuesday.
Raonic, the top seed at the ATP 250 tournament, cruised past Tim Smyczek 6-1 6-4 in just 56 minutes.
The Canadian served 10 aces and faced four break points, with Smyczek unable to convert one.
Del Potro, making his first ATP appearance of the year, was too good for 2012 champion Kevin Anderson 6-4 6-4.
Winner in 2011, seventh seed Del Potro sent down five aces and broke twice in his victory.
In a repeat of the 2015 final, Croatian second seed Ivo Karlovic - the champion in that year - was edged by Donald Young 7-6 (7-5) 7-6 (7-3).
American seeds Jack Sock and Steve Johnson also came through their openers.
Sock beat Radu Albot 6-4 7-6 (7-2) and Johnson crushed Stefan Kozlov 6-1 6-4.
Croatian Borna Coric eased past Santiago Giraldo 6-2 6-3 and Guillermo Garcia-Lopez beat Dustin Brown 6-3 6-3.
Australian sixth seed Bernard Tomic made another early exit, going down to Steve Darcis 3-6 6-1 6-4.
Jared Donaldson edged Mikhail Kukushkin 7-6 (7-5) 3-6 6-3, Damir Dzumhur was too good for Konstantin Kravchuk 6-3 6-4 and veteran Tommy Haas was beaten by Nikoloz Basilashvili 6-7 (4-7) 7-6 (10-8) 6-2.