Inter Miami's magic looked to fade due to weariness or opponents discovering and avoiding the tactics, but thousands of fans hoped it was temporary.
Lionel Messi provided two assists for Leonardo Campana to level the game late on, sending it into extra time, before they triumphed again on penalties.
Jorge Martínez appeared to have won the game in extra, but Yuya Kubo of FC Cincinnati scored late to tie the game.
Lionel Messi has gone undefeated in the US, and better for his millions of fans, he taught that even in his worst playing days, he needs sparks to win.
Miami aims to win its second title since Messi, Busquets, and Alba joined the third team's least impressive game and dominance after eight games in 32 days.
Luciano Acosta scored at minute 18 at the TQL Stadium in downtown Cincinnati, and Brandon Vázquez appeared to secure the score at 53.
The 108-year-old US Open Cup final will be played in Miami. He faces the winner of Real Salt Lake vs. Houston Dynamo on September 27.
Despite Gerardo 'Tata' Martino's four starting lineup alterations from Nashville SC (Leonardo Campana, Diego Gomez, Tomás Avilés, Jean Mota).