MILWAUKEE — Pete Alonso was having a rather disappointing evening in what could have been his final game for the New York Mets.

Then everything changed with one swing of the bat.

The slugger broke out of a prolonged slump by smashing a three-run homer off closer Devin Williams in the ninth inning to put the Mets up for good in a 4-2 victory over the Milwaukee Brewers on Thursday in the decisive third game of their NL Wild Card Series.

That made Alonso the first major leaguer to hit a go-ahead homer when his team was trailing in the ninth inning or later of a winner-take-all postseason game, according to OptaSTATS.

“It’s just something you practice in the backyard as a kid,” Alonso said. “You go through those scenarios as a little kid: All right, you’re in the playoffs down by a few runs. Words can’t explain. This is just unreal.”

Alonso and the Mets advanced to a best-of-five Division Series beginning Saturday in Philadelphia against the NL East champion Phillies.

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Alonso’s Defensive Miscue Nearly Costs Mets

The night could have turned out much differently for Alonso.

Milwaukee took a 2-0 lead in the seventh on back-to-back homers from pinch-hitter Jake Bauers and Sal Frelick, and was threatening to add on. The Brewers had runners at second and third with two outs when William Contreras hit a foul pop toward the stands that Alonso was unable to catch against the protective netting, giving the All-Star catcher one more chance to drive in some runs.

“It’s baseball,” Alonso said. “It’s a game of failure. Sometimes it’s not the case. Especially in these big games, you’ve got to move on to the next pitch and make a positive impact, do the best you can, stay within yourself and execute.”

The defensive play didn’t become a factor once Edwin Díaz struck out Contreras to end that inning. Alonso then came up with the Mets trailing 2-0 but threatening in the ninth.

Francisco Lindor had started the rally by walking on a 3-2 pitch. One out later, Brandon Nimmo singled to put the tying run on base for Alonso.

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Alonso’s Clutch Homer Saves Mets’ Season

The 29-year-old Alonso has gone deep 226 times in his six years with New York — including a 53-homer season as a rookie in 2019.

“He’s with Mike Piazza as one of the greatest home run hitters in Mets history,” teammate Brandon Nimmo said.

But he hadn’t done much lately. Alonso struggled late in the season and hadn’t had so much as an extra-base hit since homering on Sept. 19. Nimmo said teammates kept telling Alonso he was just one swing from turning it around.

Adding to the pressure was the unavoidable fact that Alonso is a pending free agent. When he stepped to the plate in the ninth inning, he knew it might be his final time at bat in a Mets uniform.

“And now it’s not because he did what big Pete does,” Nimmo said.

What he did was send an opposite-field shot over the right-field wall on a 3-1 changeup from Williams, a two-time NL reliever of the year.

“As soon as I hit it I was like, ‘Oh yeah, nobody’s catching that,'” Alonso said.

An excited Alonso put his fingers to his mouth in a “chef’s kiss” gesture as he rounded first base. The emotions continued spilling out as the Mets added an insurance run and then shut the door in the bottom of the ninth.

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Alonso’s Emotional Release After Clutch Homer

“No one knows until they go through it what that struggle is like,” Nimmo said. “When you’re going through the tough times and haven’t had an extra-base hit in a couple of weeks, three weeks, whatever it’s been, you’re just really trying to help the team however you possibly can, but it’s not there right now. Like I’ve told you guys before, you never know when that’s going to happen. This game is really, really hard. It can happen in the blink of an eye, and it can be really hard to get out of. And it’s hard to maintain your confidence during that.

“So the weight of emotions on him has probably been building up for this last three weeks of that. And the release of that, when you finally come through — and you come through in a gigantic way for your team — it’s hard to even put that into words. I’m sure that’s why he was so emotional.”

As Alonso spoke to reporters during the champagne-soaked celebration in the Mets’ locker room, he was handed a small pumpkin. Alonso called it his “playoff pumpkin” and explained that he and his wife had found it while visiting a farm outside Milwaukee when the Mets were playing here last weekend.

“Nothing’s more fall than playoff baseball and pumpkins,” Alonso said.

Thanks to his clutch homer, the Mets might keep playing all the way through Halloween.