The sixth-seeded Lopez had to rally after losing the first set to overcome Sam Querrey 4-6, 6-2, 6-3 at Delray Beach Tennis Center, improving to 4-2 lifetime against the American.
In Monday's only other match, Mannarino upended 21-year-old American wild card Jack Sock 6-4, 6-2.
Top seeds Tommy Haas of Germany and John Isner of the United States will each take the court Tuesday. Haas will face American Wayne Odesnik while Isner will take on countryman Michael Russell in the nightcap.
Swim Week Miami Beach 2014 [PHOTOS]