The 25-year-old Medvedev, ranked second in the world, has 11 ATP Tour victories, winning this year in Mallorca and Marseille. The 36-year-old Isner won two weeks ago in Atlanta for his 16th career title.
Opelka saved the lone break point he faced, had 17 aces and won 77% of his first-serve points to finish off Tsitsipas in 2 hours, 32 minutes. Opelka ended the match with one last blast of a serve that Tsitsipas simply couldn't corral.
"I really was clutch with not only my serve but my volleys in the big moments,'' said Opelka, ranked 32nd in the world.
He has two tour victories, the last in 2020 at Delray Beach.
Tsitsipas has a tour-leading 45 victories this year, winning clay-court titles in Lyon and Monte Carlo.
"It was played on the details and he prevailed,'' Tsitsipas said. "It just didn't go my way when it had to. And it's all right. I feel like there's hope for next time.''