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.
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