The American player stunned the 2021 Eastbourne Open champion with a 6-7(5), 6-4, 6-2 victory in two hours and 17 minutes.
Making his third appearance at the event, Alex de Minaur found an inspired Steve Johnson on full display at the John Harris Court on Wednesday.
And despite putting up a hard battle with breathtaking rallies included, the Aussie could not stop ranked No. 81 from advancing to the third round.
The match marked the 22-year-old return to action on tour after his early exit in Wimbledon, followed by a positive COVID-19 test result, which prevented world No. 18 from competing in the Olympics Games representing his country.
Johnson hit seven aces, winning 74 percent of his first-service points, compared to his rival 71 percent.
The 31-year-old will next face Lithuanian player Ricardas Berankis, who defeated Serbia’s Miomir Kecmanovic in straight sets on Tuesday.
Later in the afternoon (local time), Italy’s Jannik Sinner made a successful debut in Washington as he outlasted Finnish player Emil Ruusuvuori 6-2 6-4 in one hour and 18 minutes.
In an on-court interview, Sinner commented about the match: “It was not easy, obviously.
“We had some great battles, he is a very, very good player, a young guy, and I am very happy about my performance today.”
The Italian moved past Ruusuvuori without facing a breakpoint.
Furthermore, he fired three aces, winning 85 percent of his first-service points to the 22-year-old Finn 62 percent.
World No. 24 moves into the third round of the competition, where the 19-year-old will meet Sebastian Korda.
The 2021 Great Ocean Road Open champion next adversary secured his spot after beating ranked No. 61 Vasek Pospisil in straight sets on Tuesday.
At the time of writing, top seed Rafael Nadal took the first set 6-2 against Jack Sock in his first appearance at the North American venue.
The last time they played, the Spaniard edged the American player 6-3, 3-6, 11-9 at the Laver Cup in 2017, Prague.
The 2021 Citi Open is held at full 100% fan capacity from July 31-8 August.