MIAMI — For all the volatility that has plagued Jacob deGrom this season, he has pitched like an ace. So, the Mets exuded confidence Saturday night, even with deGrom matched up against the Marlins’ Jose Fernandez, who has been almost impossible to beat in his own home park.

But it wouldn’t take long to find out that deGrom’s roller coaster season had taken another dip. In a 7-2 loss to the Marlins, the Mets’ talented righthander followed one of the best starts of his career with one of his worst, his velocity dipping as the night went on.

“I wasn’t very good today and pretty much everything I threw seemed to go down the middle,” said deGrom, who left trailing 4-2 after only 3 2⁄3 innings, the second-shortest start of his career. He’d be charged with five runs and 10 hits — both season highs.

His fastball velocity appeared down much of the night, ranging from 91-94 mph and hovering in the low end in his final inning.

“He’s probably tired,” Terry Collins said. “Those are stressful innings from the start until he came out of the game, so I’m really not concerned.”

DeGrom, who totaled 94 pitches, didn’t dispute it. “It doesn’t help when you throw 30 pitches an inning,” he said.

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The lowlights included a pair of hits allowed to his counterpart Fernandez and a monstrous two-run homer by Giancarlo Stanton, whose third-inning blast traveled an estimated 441 feet and hit off the videoboard that is situated behind the leftfield foul pole.

In the fourth, when Stanton singled for the third of his four hits, Collins had seen enough.

“He didn’t have command of any of his pitches,” he said. “A lot of balls over the plate, a lot of balls up. Very uncharacteristic.”

With the loss, the third-place Mets (51-45) trail the Marlins by 1 1⁄2 games for the second wild-card spot. They are six back of the Nationals in the NL East.

The Mets will have one more shot to gain ground in Sunday’s series finale before beginning a three-game set against the Cardinals, who are a half-game ahead of the Mets in the wild-card race.

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DeGrom began the night with a 2.38 ERA, fourth in the National League. He had settled down as the season progressed despite a velocity that only slowly improved — perhaps a residual effect from the workload he carried last season as he helped the Mets to the pennant.

The Mets hoped that deGrom had moved past those concerns. In his previous three July starts, he was 3-0 with a 1.29 ERA, capping off that stretch with his one-hitter against the Phillies on Sunday. In his first career complete game and shutout, deGrom’s fastball hummed in the mid-to-high 90s, as it had last season.

But deGrom took a step backward.

The Mets led 2-1 in the third on an RBI single by Yoenis Cespedes and a sacrifice fly by James Loney, the only damage they’d do against Fernandez.

Stanton’s mammoth two-run shot off deGrom’s hanging curveball in the third gave the Marlins the lead for good.

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In the fifth, with the Mets down 5-2, Asdrubal Cabrera struck out to leave the bases loaded. It was the second time he failed to deliver a hit with runners in scoring position, making him hitless in his last 31 at-bats in those situations.

“It’s really weird,” said Cabrera, who offered little explanation for his struggles. “You don’t feel good. It’s hard for me . . . This game is not easy and all I can do is keep working hard and try to get better.”

Fernandez improved to 26-1 lifetime at Marlins Park, where his ERA is 1.47.

“You’ve got to take advantage of mistakes,” Collins said. “He fell behind in counts and we got some balls to hit. You looked up, and he’s still standing out there in the seventh.”