The 24 year old Tsitsipas had topped the world number one Daniil Medvedev of Russia in the semifinals and raced to a 4-1 set one lead versus Coric.
However in the tiebreaker the Greek struck a doublefault and two forehand errors while Coric ripped two forehand winners as well as a crosscourt backhand winner which he followed up with a 125 miles per hour ace down the tee.
Coric did not lose a point in the tiebreaker, going a perfect 7-0.
Tsitsipas left the tennis court briefly following the tiebreaker and upon his return quickly went down a break of serve before falling behind 2-4.
Coric then held his next service game after striking a 112 miles per hour ace out wide to set up game point.
Tsitsipas sliced a crosscourt backhand long to end the ten minutes long game and trail 2-5.
Then serving to stay in the match, the Greek committed three backhand errors before striking a crosscourt forehand wide to drop his service game and the match after one hour and 57 minutes of play.
Coric, who hit 29 winners against Tsitsipas moved Monday in the professional tennis rankings from number
152 to number 29 to ensure that he will be among the 32 seeds at next week's US Open grand slam tennis tournament in New York City.
The Croatian also became the lowest ranked ATP Masters 1000 level tournament champion ever following his five victories versus top 20 ranked players last week.
In 2013 Coric won the junior US Open tennis tournament en route to becoming the number one ranked boys' junior player.
Then in 2020 Coric reached the US Open quarterfinals in his best ever grand slam showing before losing to the 2020 Olympic gold medalist Alexander Zverev of Germany 1-6, 7-6 (7-5), 7-6 (7-1), 6-3.