NEW YORK — If the Mets want to go to the postseason, they need outfielder Brandon Nimmo to step up for the next month and a half.

Nimmo has stumbled since the All-Star break, entering Friday’s action hitting .140 without a home run in the second half, but his fortunes may have changed for the better. His three-run homer capped a six-run fourth inning as the Mets defeated the Marlins, 7-3, in the series opener at Citi Field.

New York was down, 2-1, when it rallied to take the lead against Marlins right-hander Roddery Muñoz. Jeff McNeil got the big inning started with a go-ahead two-run homer after Francisco Alvarez drew a one-out walk.

Muñoz couldn’t record an out after that, allowing another walk and an RBI triple to Francisco Lindor. Miami then brought in right-hander George Soriano, who hit Mark Vientos with a pitch. That brought Nimmo to the plate, and he swung at a first-pitch slider and crushed it to the second deck in right field for his first long ball of the second half.