Lopez, who has struggled for form this season, relied heavily on his serve, dominating on his own delivery and serving 13 aces as he sealed victory in a little under two hours.
In the last eight in Houston he will face Argentina's Carlos Berlocq, who breezed past Bjorn Phau 6-1 6-0.
Two seeds bowed out in last-32 action as both Alex Bogomolov Jr and Ivo Karlovic were beaten.
Seventh seed Bogomolov slipped up against teenager Ryan Harrison, losing 4-6 6-3 6-4, while eighth seed Karlovic was beaten 7-6 (7/5) 2-6 6-3 by another American, Bobby Reynolds.
There were no such issues for fifth seed Kevin Anderson, who beat Joao Souza in straight sets.