Mets leave no doubt in NLCS Game 5 victory, plant good vibes as they head to L.A.

The Mets showed up at Citi Field for Friday night’s NLCS Game 5 with the desperate intent of keeping their remarkable October alive.

Backs to the wall, down 3-1 to the Dodgers, the theme going in was survival.

You’d never have known it.

Starting with the Temptations’ performance of “My Girl” — as the smiling Francisco Lindor sang along with the sellout crowd of 43,841 — the Flushing ballpark was the happiest it’s been all season despite hosting the most pressure-packed, anxiety-ridden game of the year.

Or to use the words Pete Alonso has said numerous times about these Mets, “The vibe was immaculate.”

Matt Harvey threw the ceremonial first pitch to Yoenis Cespedes. A towel-waving Mookie Wilson fired up the fans with “Let’s Go Mets!” chants.

And then the real fun started.

With Alonso potentially playing in his final game at Citi Field, he was determined to at least keep the Mets’ uniform on for another day, slugging a three-run homer in the first inning that served as the party-starter in a 12-6 victory over the Dodgers that sends this NLCS to Chavez Ravine.

The Mets pounded out 14 hits, rode starter David Peterson into the fourth inning, then relied on Reed Garrett, Ryne Stanek and finally Edwin Diaz, who entered to his usual trumpet-blaring light show and closed out the Dodgers for two innings.

Friday’s vibe brought flashbacks of the Mets’ Sept. 22 regular-season finale at Citi Field, when the Mets, to a man, expressed a sense that they still had more baseball to play in Flushing this year. To do it, of course, the Mets not only had to qualify for the playoffs, but win the subsequent wild-card series, which wound up being against the NL Central champ Brewers in Milwaukee.

They did both, returning home as promised, and dispatched the hated Phillies to earn the first clinching celebration in the 16-year history of Citi Field.

We’re getting those same feelings again after watching the Mets step back from the playoff abyss, and do a little victory dance. The Dodgers figured to be the favorite after outscoring them 30-9 in the first four NLCS games and bringing back Jack Flaherty, who allowed only two hits over seven scoreless innings in the Game 1 blowout.

But the Mets seem to function best with the odds against them, and they jumped all over Flaherty, hammering him for eight hits and eight runs in three innings. What was the difference this time? Many of the Mets had previously cited the three-day layoff between the Division Series clincher and NLCS opener as reason for a little rust, and that was certainly believable in how they dismantled Flaherty during Friday’s brief mound stay.

“We had three days off without seeing pitching,” Mark Vientos said before Game 5. “So we were kind of a little off at the plate.”

That was not the case Friday. In the first inning, with two on, Flaherty threw an 85-mph slider to Alonso that he probably thought was in a safe space, only a foot above the plate. But Alonso reached down and somehow golfed a 432-foot homer into the blacked-out batter’s-eye in centerfield (a member of the orange-clad 7 Line Army actually stretched for a nifty one-handed grab).

Alonso probably figured he was due for another memorable blast, and what better time or place? He saved the Mets’ season earlier this month with the ninth-inning homer that beat the Brewers in Game 3 of the wild-card round, and even made his playoff mark at Citi by homering off the Phillies in the Division Series.

With the Mets facing elimination again, just like that night in Milwaukee, Alonso came up big when it mattered most. And on Friday, Alonso spurred the rest of his teammates, too, as they piled on to create more distance between them and the Dodgers.

The Mets sent nine batters to the plate and scored five runs in the third inning, the major blow delivered by Starling Marte’s two-run double. Francisco Alvarez, once slumping but now impossible to get out, smacked an RBI single and the hobbled Brandon Nimmo followed up a Lindor RBI triple with a run-scoring single of his own to put the Mets ahead 8-1.

As the hits and runs stacked up, the Citi Field roars grew louder, the crowd not only believing in Friday’s runaway victory, but maybe even the chance of the Mets giving them more games in Flushing this month. And why not? If the Mets could summon the resolve to steamroll the Dodgers after being smacked around for most of this NLCS, they can definitely win two more games in L.A.

For Sunday’s Game 6, they’ll send out Sean Manaea — who was solid before being pulled in the sixth inning of the Mets’ 7-3 victory in Game 2 — with Luis Severino lined up to start an all-hands-on-deck Game 7 on Monday night. It’s a tall task.

But one thing we’ve learned about these Mets: Anything is possible. They’ve been preaching that mantra since Memorial Day. And if they grind out two more wins at Chavez Ravine, the Mets will be in the World Series, and back to Citi Field, where the party never stopped Friday night.

By David Lennon

dave.lennon@newsday.comDPLennon

David Lennon is an award-winning columnist, a voter for baseball's Hall of Fame and has covered six no-hitters, including two perfect games.