The towering Opelka was forced to retire in the first set due to a right knee injury, granting his Greek opponent passage to the semifinals. Tsitsipas had never moved past the first round in Cincinnati, and will now be competing in his fifth ATP Masters 1000 semifinal.

At 4-3 in the first set, Opelka required a medical timeout to have his right knee taped. The unseeded American attempted to fight through the pain, but eventually threw in the towel at 6-5.

Tsitsipas next faces the winner between the resurgent Milos Raonic and in-form Serbian Filip Krajinovic for a place in the final. Although the Greek star lost his most recent match against Raonic at this year's Australian Open, he does hold a 2-0 career lead over Krajinovic.

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