roared as the two players reached a decisive tiebreaker.
Early in the tiebreaker Tsitsipas struck three aces to post his only points won.
Alcaraz meanwhile mixed soft dropshots and powerful forehand winners to reach matchpoint at 6-3.
Alcaraz struck a crosscourt backhand wide to drop his first match point before striking a down the line forehand lob just long to reach 6-5.
However he made no mistake on his third matchpoint; striking a crosscourt inside out forehand winner before falling on his back on the court in celebration of the incredible upset after four hours and seven minutes of play.
It was the 61st winner of the day for the young Spaniard who became the youngest male player to beat a top three player in US Open open era history.
Later yesterday in a late night match, the 23 year old Frances Tiafoe faced the same aged 5th seeded Andrey Rublev of Russia in front of a loud home crowd.
After striking extremely powerful serves and groundstrokes, Tiafoe raced to a 5-0 lead in set five.
With Rublev serving to stay in the match at 15 all, the Russian struck an ace out wide to lead 30-15 before holding serve to reach 5-1.
Serving for the match, Tiafoe struck a 131 miles per hour ace down the tee to lead 15-0.
Rublev netted two consecutive forehands to face matchpoint.
Tiafoe struck a 128 miles per hour ace down the tee; his 24th ace and 70th winner of the night, to end the match after 3 hours and 46 minutes of play.
The match finished at 2:14am local time for one of the top ten latest US Open finishes ever.
In 2013, Tiafoe won the Orange Bowl junior tournament at age 15.