Fritz, who reached last week's Los Cabos Open semi-finals and is currently ranked No.36 in the world, edged out Opelka 7-6 (7-5) 6-7 (3-7) 7-6 (7-4) in just less than three hours.
The pair sent down a combined 48 aces, with Fritz claiming 25 of them, with all sets going to tiebreakers, with no breaks of serve.
No.3 seed Cameron Norrie was bundled out by Finnish world No.78 Emil Ruusuvuori in straight sets.
Norrie, who won last week's Los Cabos Open for his maiden ATP Tour title and beat Australia's Nick Kyrgios here, was beaten 7-6 (7-1) 6-3 by the Finn.
Ruusuvuori will take on teenager Brandon Nakashima in the semi-finals, after the American, who was runner-up to Norrie in Los Cabos, beat Jordan Thompson 7-6 (7-5) 7-5.
No.6 seed John Isner sent down 30 aces as he got past Australian qualifier Chris O'Connell 7-6 (7-5) 6-7 (3-7) 6-4.