The Canadian No. 7 seed, who enjoyed a breakthrough year in 2018, looked comfortable as he broke Sousa twice to win the opening set 6-4.
Sousa responded to take a 4-1 lead in the second, holding serve in the final game of the set to win 6-4 and level the match. The Portuguese then sealed the only break of serve in the decider to win in two hours, 22 minutes.
Matteo Berrettini defeated another U.S. qualifier in Mackenzie McDonald 6-3, 6-4, setting up a second-round clash with German Maximilian Marterer who defeated Tennys Sandgren 6-3, 6-4, while Jan-Lennard Struff advanced after Serbian opponent Laslo Djere retired in the second set with the German leading 7-6 (5), 2-0.