The big-serving American dropped the opener on a tie-break before recovering to clinch a 6-7, 6-4, 6-4 win over the fourth seed at the Crandon Park Tennis Centre.

"I couldn't have scripted this," said Isner. 

"I came into this tournament, I had won one ATP World Tour match all year and was playing very poorly.

"I won my first match in three sets and that's how tennis goes - you start to gain a little confidence and next thing you know things start to roll your way."

The first set was a tense affair as both players held serve throughout before Zverev, the world number five, moved to within one set of victory on the tie-breaker.

In front of a partisan crowd, Isner began to find his form in the second and broke the German on his way to levelling the match at one set all.

Isner, who had lost his previous two finals at this level, produced a dominant serving performance in the final set as he faced just three break points.

Zverev was unable to match the American and eventually succumbed to defeat.

The match was the last to be held at the venue is Key Biscayne as the tournament will move to a new home from next year.