NEW YORK -- Even though this Mets team is undeniably different from its recent predecessors, with more realistic postseason aspirations, the fog of past seasons can be difficult to disperse. So when the Yankees strutted into Citi Field on Tuesday and immediately hammered consecutive homers over the fence, the Bronx portion of a sold-out crowd made its confidence known.

It was loud and it was antagonistic, until the Mets did something they’ve done so infrequently over the decades-long run of this rivalry: They punched back.

Four of the first six Mets batters clubbed extra-base hits in the bottom of the first inning, including a solo homer from Starling Marte and a go-ahead two-run shot by Eduardo Escobar. The resulting lead held through the middle innings and onto the end game, when Edwin Díaz nailed down the final four outs of a 6-3 win over the Yankees.

“I’ve never played in a Major League playoff game before,” first baseman Pete Alonso said, “but if I had to guess, that’s what it would be like.”

Heading into the night, such an atmosphere was exactly what the Mets had hoped to engender in their home borough. Since 1997, fans have circled the Subway Series on their calendars, selling out stadiums in the Bronx and Queens. Most years, however, those matchups offered little more than bragging rights; only rarely did the Mets and Yankees both enter a Subway Series with playoff aspirations.

This year promised to be different, marking the first time the interborough rivals both began a Subway Series in sole possession of first place. It was always going to be loud. It was always going to be a big deal to the 42,364 fans who crammed into Citi Field, and to the multitudes more who watched on television. When Aaron Judge and Anthony Rizzo homered in the first inning off Taijuan Walker, it was always going to seem like the Yankees were bullying their kid brother again.

Only this time, the kid brother would have none of it. After Brandon Nimmo opened the bottom of the first with a nine-pitch at-bat against Yankees starter Jordan Montgomery, Marte homered to shift both the atmosphere and the perspective. Next up were Francisco Lindor and Alonso, who followed with back-to-back doubles, setting up Escobar’s go-ahead homer over the left-field fence.

At that point, half the crowd quieted. The other half ignited

“It was honestly like a World Series home run,” said Escobar, a 12-year veteran with multiple postseason appearances. “I’ve never played in an environment like that.”

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From there, manager Buck Showalter managed similarly to how he might in a playoff game. After Walker recovered to take the Mets through six innings, Showalter asked Adam Ottavino for five outs. He turned from there to Díaz for the final four, which his closer recorded exclusively via strikeout. (Díaz has recorded 27 of his last 36 outs in that manner.)

“They just played better than us tonight,” Yankees shortstop Isiah Kiner-Falefa said.

In the postgame clubhouse, Mets players didn’t bother downplaying the significance of the matchup, knowing it was a worthy test for them. More than 60 media members had crowded into the room before the game, transforming even routine tasks into an obstacle course. Mets great Darryl Strawberry threw out a ceremonial first pitch. Even at that early hour, the stands were nearly full of fans, who, in Marte’s words, “really brought it tonight.”

When asked about the series’ importance, Alonso explained matter-of-factly that the Yankees “are the best team in the American League,” adding that if the Mets can beat teams like that, “we’re going to end up where we want to be.”

Tuesday proved that the Mets are indeed capable of doing so, if any doubt still existed. An early deficit was never going to alter their confidence, nor dispel the notion that the kid brother came to fight.

“It’s like I told the players in the advance meeting: I said they should relish playing in this environment and have fun, draw something from it,” Showalter said. “We’ll be able to reach back for it as we go forward.”