Jannik Sinner worked for nearly three hours to win the biggest title of his career with a 7-5, 4-6, 7-5 defeat of American Mackenzie McDonald in the Washington final on Sunday night.
The Italian who turns 20 in a week had to fight past his opponent, a former US college champion, in the evening heat of the US capital.
Sinner added the ATP 500 honour to the 2019 NextGen title and a pair of 250 trophies in Melbourne and Sofia.
The modest winner, from the German-speaking mountain north of Italy, did not dwell on his success: “There is still work to be done. I need a lot more experience,” he said.
“I will keep working hard as we are doing now, trying to play the important moments in matches.
“Today I had a lot of them. I can learn many things (from) today.”
McDonald, ranked 107, put on a strong display, saving 10 set points in the opener and holding off 16 of 21 break points during the final
Teenaged Sinner became the third-youngest Washington winner after Andy Roddick (2001) and Juan Martin del Potro (2008).
“When you see somebody is the youngest or whatever, I don’t put much weight on that,” the 19-year-old said.
“There are a lot of players who have done much, much better than me. It’s not about who is the youngest or whatever. I just want to improve, work hard.”
The Italian is the first teenager to lift a trophy at the 500 level – he lost just one set over five matches – and now heads to the Toronto Masters with a bye as 16th seed.