Hewitt, once ranked No. 1 in the world, recorded a 6-4, 6-3 victory that set up a semifinal meeting with Rajeev Ram.
Ram, a former winner of this event, surprised No. 2 seed Kei Nishikori 7-6 (8-6), 6-3 in their quarterfinal match.
In second-round action Thursday, Benjamin Becker ousted Milos Raonic 6-3, 6-3, and No. 6 seed Ryan Harrison defeated Jesse Levine 6-3, 6-4.
The other quarterfinals will be held Friday, with defending champion John Isner facing Izak Van der Merwe and Harrison taking on Becker.