Fifth seed Jurgen Melzer was dumped out of the Abierto Mexicano Telcel in Acapulco after losing 6-4 6-3 to Joao Sousa in the first round.
Despite being ranked 44 places below his opponent, world 93 Sousa was on top throughout as he completed victory in 1 hour and 23 minutes.
Portugal's Sousa won 75 per cent of the points on his first serve and broke his Austrian opponent on four occasions.
Elsewhere, Spain's Pablo Andujar got just his second win of the season as he beat Dusan Lajovic of Serbia 3-6 7-5 6-2.
There were wins for Colombia's Santiago Giraldo, who will face Sousa in the next round, and Argentinian pair Leonardo Mayer and Martin Alund.