The Marlins counted on two rookies -- right-hander Kyle Barraclough and lefty Raudel Lazo -- to navigate through the sixth.
Bryan Morris was brilliant over two perfect innings, the seventh and eighth, striking out three. And A.J. Ramos locked down his 27th save in the ninth, stranding Anthony Rendon at third after he opened the inning with a double and reached third on an error.
Ramos responded after Rendon's hit by striking out Danny Espinosa, Bryce Harper and Jayson Werth in order.
The Harper showdown was key as Ramos got the slugger to offer at some filthy sliders.
"I definitely didn't want to get beat by him," Ramos said. "That's the guy you don't let beat you. I made sure I threw the nastiest pitches and hoped he'd bite on them. My slider was working really well today and he was swinging at it. Just use that. I was confident with that for the rest of the hitters."
In the sixth inning, the Marlins' bullpen picked up another momentum-swinging out, when Lazo got Harper to lift a fly ball to right with two outs and Trea Turner on third.
"Lazo gets the big out with Harper there," Jennings said. "Then B-Mo, two innings of just total domination there, face six guys with three punch-outs. A.J. gets No. 27. Night after night the bullpen steps up in a huge way.
"They've stepped up in a huge way. They are making a name for themselves. They're performing in the opportunities and the roles they're put in."
The efforts of the relievers secured the victory for Fernandez, who was making his first start since Aug. 7 as he returned from a right biceps strain.
"You see, when [Fernandez] pitches, the team is different, it seems like," Ramos said. "He brings something to this team that nobody can explain. Certain people have that factor, and he's one of those guys. It's uplifting for the team."