Playing in his first US Open main draw, world number 128 Stricker used a massive serve, booming forehand and solid net play during a gruelling four-hour encounter in which he broke the Australian Open finalist six times for his first top-10 win.
"I came out today pretty well. I felt good from the first set on. It was a tough battle but I am just super happy right now," said Stricker. "I am going to enjoy the rest of the day and then I will recover for the next round."
For Tsitsipas, who won an ATP 250 title in Los Cabos in early August but then went a combined 1-2 at Masters 1000 events in Toronto and Cincinnati, the loss marked another early US Open exit after falling in the first round last year.
Tsitsipas had a chance to serve for the match while leading 5-3 in the fourth set but Stricker stepped up with the break en route to forcing a decider where he consolidated an early break with a hold to love for a 3-0 lead that sent him on his way.
"I was down 3-5 and then I came back in the fourth set. I don't know how, but I did it somehow and then I kept playing very high level tennis. I am a bit speechless but it is a great day," said Stricker.
"Such a great day for me, such a great win," Stricker added. "It gives me a lot of confidence and I think it is great to see from my team. We are working hard every day and it is so great to do that."
