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The big-serving American had lost his five previous ATP meetings against De Minaur and was trying to avoid falling to the same opponent on six occasions for the first time. In hot conditions, the 27 year-old did just that, taking large cuts at the ball on return and saving all seven break points he faced to record his ninth Top 10 win after one hour and 41 minutes.

"I didn't serve as well as I'd have liked to, but everything else was working," said Opelka, who won 76 per cent of his first-serve points and 69 per cent of his second-serve points. "My forehand helped me a lot and my backhand, playing with a lot of offense and I was pretty consistent from the baseline the whole match. 

"My serve fluctuated and it was a bit of a surprise that the other things came through well for me today. I think I returned really well too."

World number-eight De Minaur arrived in Cincinnati having earned a Tour-leading 24 hard-court wins this season, highlighted by his recent title triumph in Washington. However, he was unable to convert his opportunities against Opelka, who struck a huge forehand winner on return at 5/5 in the first-set tie-break en route to claiming the opener.

Opelka is up nine spots to No. 64 in the ATP Live Rankings and will next meet Francisco Comesana after the Argentine advanced on Sunday when 29th seed Luciano Darderi retired trailing 4-6, 1-3.

Opelka has produced some of his best tennis on the North American hard courts in the past, reaching the final at the ATP Masters 1000 event in Toronto in 2021 and the fourth round at the US Open in the same year. He clubbed 38 winners, including 14 aces, to De Minaur's six winners, with the Australian frequently returning second serves above shoulder height.

In other early action, ninth seed Andrey Rublev moved past American Learner Tien 7-6(4), 6-3 to reach the third round. Rublev arrived in Cincinnati off the back of a quarter-final showing in Toronto, his best ATP Masters 1000 result of the year. 

The No. 11 in the  ATP Rankings will meet Alexei Popyrin or Spaniard Martin Landaluce in the last 32.

There was Czech success for Jakub Mensik and Jiri Lehecka. Miami champion Mensik, 19, defeated American Ethan Quinn 6-4, 6-2 to earn his first win at the hard-court Masters 1000 event. 

Lehecka rallied past American Tristan Boyer 4-6, 6-1, 6-4 to reach the third round. 

The Czech advanced to the last 16 in Ohio in 2024 and will aim to reach that stage again when he takes on Adam Walton.