Messi delivered two exquisite crosses onto the head of striker Leonardo Campana, the second with just a minute of stoppage time remaining, to rescue his team and force extra time.
Substitute Josef Martínez looked to have completed the turnaround with a cool finish in the third minute of the added 30, but Cincinnati refused to lay down and made it 3-3 through Yuya Kubo to lead to a penalty shootout.
It was Miami’s second shootout in four days, after beating Nashville SC in the Leagues Cup final on Saturday to lift its first trophy in club history.
Messi scored from the spot, but once again it was goalkeeper Drake Callender who made himself the hero, producing the only save of the shootout from Nick Hagglund before Benjamin Cremaschi fired home the winning kick.
Miami can now look forward to another final, against either the Houston Dynamo and Real Salt Lake, next month. Meanwhile, a shell-shocked Cincinnati, which had looked set to reach its first-ever final, becomes the latest victim of an extraordinary Miami run since Messi landed in South Florida.
Messi had scored 10 goals in seven games to lead Miami to the Leagues Cup trophy. It was another No. 10 from Argentina, though, who struck the first blow at TQL Stadium on Wednesday.
Luciano Acosta, who sits second on MLS‘ scoring charts, made Cincinnati the first opponent to score the opening goal against a Messi-led Miami team after poking the ball in off the post in the 18th minute.
Cincinnati doubled its advantage eight minutes into the second half. U.S. international striker Brandon Vázquez produced the kind of pinpoint, emphatic finish Messi would have been proud of to put the finishing touch on a sweeping breakaway.
At that point, there looked no way back for the visitors against a Cincinnati team with the best record in MLS this season.
The seven games in a month that led Miami to an unexpected trophy had also taken their toll on the roster, leading coach Gerardo “Tata” Martino to make four changes to his starting lineup, along with a formation switch, as it looked to reach another final.
Miami struggled for much of the contest and managed just one shot on target — set up by Messi — in the first 65 minutes. A triple substitution by Martino sparked some life into his team, but it was Messi again who provided the crucial contributions.
The seven-time Ballon d’Or winner was held scoreless for the first time in a Miami jersey, but this time it was his assists that did the damage.
First, he delivered a perfect free kick that Campana headed home to bring the score back to 2-1 and give Miami hope in the 68th minute.
But as the clock ticked past 90 minutes, it looked like Messi was about to taste his first defeat as a Miami player. Instead, the World Cup winner had other ideas, supplying another wondrous left-footed cross that Campana stooped to head into the net in the seventh of eight added minutes.
The tide had turned, and Martinez’s early goal in extra time looked to have delivered the final blow to Cincinnati. A passionate home crowd that had delighted in booing Messi’s every touch were given hope when Cincinnati leveled the game once more. But Miami would not be denied.
Messi will now grace the final of a competition first contested in 1914 and that stands as the oldest active sports tournament in the United States and one of the oldest annual soccer competitions in the world.
Before that, though, Miami will have to confront its pre-Messi reality when it resumes MLS regular-season play with the worst record in the league and an uphill task to make the playoffs. Miami will visit the New York Red Bulls on Saturday, when Messi is likely to make his MLS debut.