Fritz defeated fellow American and third-seeded Jack Sock 7-6 (6), 6-2 after hitting eight aces at the clay-court event, a warmup tournament for the French Open which starts on Sunday at Roland Garros.
Fritz will next take on Dusan Lajovic of Serbia for a semifinal spot. Lajovic defeated qualifier Filip Horansky 6-4, 7-5.
The second-seeded Isner saved a set point in the opening set and broke once in the second to prevail 7-6 (3), 6-3 against Radu Albot. Next for Isner will be Cameron Norrie of Britain.