The win will enable Isner to become the highest-ranked American at No. 9 when the new world rankings are released on Monday. "I came into this tournament, I had won one ATP match all year and was playing very poorly".
The moment inspired Isner, who changed to a yellow shirt and won that set and the next for his first victory over World No. 5 Zverev. "I won my first match (here) in three sets and that's how tennis goes - you start to gain a little confidence and the next thing you know things start to roll your way", said the 14th seeded Isner during his courtside interview, via Reuters.
He played an nearly flawless semifinal to beat Juan Martin del Potro, and was just good enough against the fourth-seeded Zverev, a precocious 20-year-old German who was bidding for his third Masters 1000 title in the past 12 months. "I'm toward the latter part of my career; this is the best moment of my career".
Isner is the first U.S. player to lift the Miami Open title since Andy Roddick in 2010.
Isner's win solidified a sweep by American players throughout the tournament, with Sloane Stephens winning the women's singles, Bob and Mike Bryan taking the men's doubles, and CoCo Vandeweghe winning with Australian Ashleigh Barty in the women's doubles.
"I was just ready for this moment. I was just ready for it".
The Key Biscayne finale drew a near-capacity crowd, and the atmosphere was electrifying at times.
Isner started his campaign in Miami with an up and down three-set win over Jiri Vesely, but then didn't drop another set until losing the first on a tiebreak to Zverev in the final.
Isner's serve allowed him to overcome inconsistent returning, a succession of botched volleys and shaky play at crunch time. Re-energised, the American saved two break points in the next game to take the set.
The young German was defeated by John Isner but he gathered respectable 600 points that propelled him into the leading spot in the U21 Masters standings after the opening three months of the season. "I will be a force if I can continue to play freely".
Mindful of that small margin for error when he lost serve to fall behind 5-4 in the final set, Zverev let his frustration boil over.