Sousa sped past the Austria's Melzer 6-4, 6-3 on the clay courts at the Fairmont Acapulco Princess.
Spain's Pablo Andujar, however, fared better, moving on to the second round with a 3-6, 7-5, 6-2 victory over Serbian Dusan Lajovic.
Other Day 1 Acapulco winners Monday included Martin Alund of Argentina, Colombian Santiago Giraldo and Leonardo Mayer of Argentina.
Members of Congress to keep receiving porn magazine
Putin thinks Obama would save him if he were drowning