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The 26th-seeded Hurkacz, who had eliminated four higher-ranked players en route to the final, held his nerve at Miami's Hard Rock Stadium to win 7-6 (7/4), 6-4 in 1hr 43min.
It was a deserved victory for the 24-year-old from Wroclaw, who claimed his second title of the season as Sinner's challenge unravelled with a series of unforced errors.
"It feels unreal, so happy that I was able to play a good match today," Hurkacz said.
"Jannik is an amazing competitor. I'm so proud of myself that I was able to beat him."
Hurkacz, whose win earned him a message of congratulations Polish President Andrzej Duda, said he hoped the tournament win would be a launchpad for more victories.
"It gives me extra motivation and extra belief in myself, that I've won a really big title," he said. "It motivates me to be a better player so I can win even more."
Sinner meanwhile expressed disappointment that he had been unable to take full advantage of the absence in Miami of the men's "Big Three" -- Novak Djokovic, Rafael Nadal and Roger Federer -- by crowning a successful tournament with victory in the final.
"Today is a tough day, to be honest, because losing the final here for me is not easy," Sinner said.
"I came here with the mentality to win. We knew that Rafa, Roger, Novak were not playing," he said. "I think it was a big opportunity for many players."
Sinner had started slowly, being broken with his first service game to quickly find himself trailing 3-0 in the opening set.
However the talented 19-year-old grew in confidence as the set progressed, working Hurkacz around the court with some pinpoint forehands into the corners.
The Italian broke back in the fifth game and then held to make it 3-3, before another break of Hurkacz's serve gave him a 6-5 lead and the chance to serve for the set.
Sinner appeared to tighten on serve, though, and another unforced error, sending a backhand long, allowed Hurkacz to break back at 6-6 and force the tie break.
Hurkacz quickly raced into a 6/2 lead in the decider and although Sinner clawed it back to 6/4, another missed forehand handed Hurkacz the first set.
Hurkacz carried that momentum into the second set, sprinting into a 4-0 lead with a double break.
With the match slipping away from him, Sinner rallied to pull the score back to 4-3 but was unable to break an increasingly jittery Hurkacz in the eighth game to leave the Pole 5-3 up.
After Sinner held for 5-4, Hurkacz once again benefited from unforced errors by his opponent at key times as he served for the championship.
An overhit forehand by Sinner set up match point, and on the next point the Italian's attempted forehand down the line drifted wide to seal Hurkacz's win.