The  Mets' Pete Alonso celebrates his three-run home run during...

The  Mets' Pete Alonso celebrates his three-run home run during sixth inning at the MLB All-Star Game on Tuesday in Atlanta. Credit: AP/Brynn Anderson

ATLANTA — Nearing midnight Tuesday, Pete Alonso found himself in a Truist Park batting cage off the home clubhouse, taking swings, watching the All-Star Game broadcast on a nearby TV.

Incredibly, Alonso was preparing for a home-run contest, only this one to decide a winner of the Midsummer Classic, something officially labeled a “swing-off,” the first such tiebreaker in baseball history.

A day earlier, Alonso chose to skip the actual Home Run Derby, preferring to rest up for what figured to be a huge second half, for both him and the Mets. The decision was a bit of a shocker coming from the two-time champ, but respectable, with Alonso’s second shot at free agency looming and the Mets gunning for another deep playoff run.

But as fate would have it, Alonso was being asked to take aim for the seats again, Derby-style, after already hitting a three-run homer in the sixth inning off the Royals’ Kris Bubic that gave the NL a 5-0 lead that didn’t last (thanks to the NL bullpen capped by Edwin Diaz’s blown save in the ninth — more on that later).

“They were like, all right Pete, you’re gonna close it out,” Alonso said early Wednesday morning. “So I went down in the cage to get the body loosened up again.”

Count Alonso among the relatively select crowd at Truist Park who knew the All-Star Game would be going to a swing-off tiebreaker, a rule that had been in place for the past three years yet surprisingly few players knew about it. Three sluggers picked from each side, three swings each, most aggregate homers gets the W.

Alonso, of course, would bat third for the NL, essentially the cleanup hitter. But with the perfect Polar Bear story line in place, Alonso never got to the plate.

That’s because the Phillies’ Kyle Schwarber — with Alonso watching on TV — launched three moon shots into the Atlanta night, giving the National League a 4-3 edge before the Rays’ Jonathan Aranda came up empty in his turn, giving the NL a victory. As a result, Schwarber was handed the MVP trophy that likely would have gone to Alonso if not for those swing-off heroics.

“He put on a hell of a show,” Alonso said. “I’m watching in the cage, taking my swings, and every one I see go over the fence, ‘I’m like, hell yeah, Schwarbs!’ When he was launching balls, you could hear people from the tunnel. Everyone was super into it.”

So why did the AL wind up using Aranda, Randy Arozarena and Brent Rooker as their homer champions? Well, this being the All-Star Game, players shower, dress and bolt the ballpark shortly after they’re removed from the lineup, so Aaron Judge and Cal Raleigh — the AL’s top sluggers — didn’t figure into manager Aaron Boone’s plans when he set up his trio a day earlier. Same with Shohei Ohtani on the NL side, and manager Dave Roberts even had to sub Kyle Stowers for Eugenio Suarez, who was hurting after taking a pitch off the wrist earlier.

Under the swing-off format, each league takes turns, and Rooker led off with a pair of home runs, followed by Stowers with one, then Arozarena answered to put the AL up 3-1. The mighty Schwarber took it from there.

“That’s Schwarbs. I’m not shocked,” Boone said. “I saw him nodding his head like, I’ve got the speed on a couple of the takes, and he did Schwarber-type things.”

Schwarber wound up stealing the show, but this was the Polar Bear’s stage before that point. In the sixth, with the NL clinging to a 2-0 lead, Alonso smacked a 93-mph fastball from Bubic over the high rightfield wall, a 367-foot shot that just reached the first row of seats. Alonso became only the third Met to go deep in an All-Star Game, joining David Wright (2006) and Lee Mazzilli (1979).

“For me, I think that [homer] beats any Derby win,” Alonso said. “I’m stoked to perform the way I did today. The Derby, in that environment, it’s just batting practice at the end of the day. For me to do it, where somebody’s trying to get me out and not groove meatballs in there, it’s really special to do that.”

If not for Diaz getting tripped up in the ninth, that epic swing-off never would have materialized. The Mets’ closer was called on to replace Robert Suarez after Bobby Witt Jr’s RBI double cut the NL’s lead to 6-5 with no outs and got an incredible diving play from Matt Olson to rob Jazz Chisholm of a 105-mph potential tying hit. Still, Steven Kwan’s 54-mph infield dribbler — after fouling off two 100-mph fastballs from Diaz — made it 6-6 anyway.

That set up a final showdown with Arozarena, and Diaz preserved the tie by challenging with an 0-and-2 pitch that was called a ball. This being the first All-Star Game with the automated ball-strike system (ABS), the videoboard showed his 100-mph heater just nicked the outside edge of the zone.

“It was pretty funny,” Diaz said afterward. “I wasn’t sure it was a strike, because it was really close. As soon as I saw the strike, I just started laughing and pointing at Arozarena.”

A wild All-Star night only got crazier from there.

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