"I didn't play so good - it is maybe one of the worst defeats of my career," said Ferrer.
"Today I made a lot of mistakes like a junior."
Isner won his way through to the final after overcoming a horrible start in his semifinal showdown against Roberto Bautista Agut.
The 2010 champion looked slow and laboured around the court early on before taking the initiative mid-way through the second set to claim a 3-6 7-6(2) 6-4 win.
Dictating the pace of the game at the outset, Bautista Agut forced the 2.08m tall Isner into a running game, which he struggled with. After easily claiming the opening set Bautista Agut looked headed for his second ATP Tour final with a 5-3 lead in the second.
But Isner came up with a timely break of serve, eventually forcing a second set tie break.
With the best record in tie breaks on the tour, Isner predictably won before powering away in the third set, setting up match point with a 231km ace.
Tomorrow's doubles final has an element of symmetry to it, with two Brazilian-Austrian pairings going head-to-head.
Top seeds Alexander Peya (AUT) and Bruno Soares (BRA) won their way through to the final with a dominant performance over Italy's Daniele Bracciali and Lukas Dlouhy of the Czech Republic. They will take on the second seeds Julian Knowle (AUT) and Marcelo Melo (BRA) in the final.