Grigor Dimitrov battled to a hard-fought win in the Los Cabos Open first round, while Taylor Fritz was untroubled on Tuesday.
Dimitrov, who has fallen out of the world's top 50 after a tough first half of the year, got past Steve Johnson 7-6 (7-4) 4-6 7-6 (7-5) at the ATP 250 tournament.
The Bulgarian served 12 double faults but still managed to edge through for his first victory since the French Open.
Fritz, coming off a run to the final in Atlanta, disposed of German Dominik Koepfer 6-3 6-4 in just an hour.
The American fifth seed managed to break once in each set in what was a routine outing.
Radu Albot, the seventh seed, also managed to get through courtesy of a 7-5 2-6 6-0 win over Janko Tipsarevic.
The only seed to depart was Cristian Garin, who went down to Juan Ignacio Londero 3-6 6-2 6-3.
Tennys Sandgren was thrashed 6-2 6-0 by Taro Daniel, Cameron Norrie got past Gregoire Barrere 7-6 (7-5) 6-0 and qualifier Kwon Soon-woo easily beat Cedrik-Marcel Stebe 6-2 6-1.