PHILADELPHIA — Another almost improbable Mets victory turned into a gut punch Sunday night.

One miracle short, the Mets couldn’t survive the bottom of the ninth after Mark Vientos’ second homer of the game a half-inning earlier had his team harboring dreams of heading to Queens within a victory of reaching the NLCS.

But Tylor Megill surrendered a walk-off RBI single to Nick Castellanos in the bottom of the inning, sending the Mets to a 7-6 loss to the Phillies in Game 2 of the NLDS at Citizens Bank Park.

Megill, who walked Trea Turner and Bryce Harper, had entered the game in the eighth following an Edwin Diaz implosion.

The great wild card in this Mets surge into the postseason and early success has been which version of Diaz will show up on any given day or inning.

There was the Diaz who imploded in last Monday’s eighth inning in Atlanta before begging for the ball in the ninth and shutting the door to catapult the Mets into the playoffs. There was the Diaz who three nights later stifled the Brewers and allowed the Mets to rally for a NL Wild Card Series clinching victory.

And then there was Sunday night, when it came full circle for the right-hander. Diaz, after getting the final out in the seventh inning and one to begin the eighth, cracked again.

Bryson Stott’s two-run triple against Diaz was the backbreaker.

The Phillies’ go-ahead rally began with Harper drawing a one-out walk in the eighth against Diaz before Castellanos singled on a 98-mph fastball that put runners on the corners. Stott, on the sixth pitch of his at-bat, hammered a slider into the right-field corner for a two-run triple that put the Mets in a 5-4 hole. Diaz departed for Megill and J.T. Realmuto’s RBI fielder’s choice brought in the insurance run.

But following Francisco Lindor’s single in the ninth, Vientos went deep against Matt Strahm to tie it 6-6.

After Luis Severino flushed a 3-0 advantage by allowing consecutive homers to Harper and Castellanos in the sixth to tie it, Brandon Nimmo regained the lead for the Mets with a solo blast in the seventh.

Severino’s final line over six innings (91 pitches) included three earned runs on six hits with seven strikeouts. It was a second straight start this postseason in which the right-hander surrendered three earned runs in six innings (also in Game 1 of the NL Wild Card Series on Tuesday in Milwaukee).

Vientos hit a two-run homer against Cristopher Sanchez in the third to account for the game’s first runs. Francisco Lindor singled in the inning before Vientos went to the opposite field, just clearing the fence in right for his first career postseason homer.

Starling Marte’s throwing error in the third allowed Johan Rojas to advance to second on Kyle Schwarber’s fly out. Rojas stole third but was left stranded. Turner stole second and third in the first inning for the Phillies before Severino struck out Harper and retired Castellanos.

Stott singled with two outs in the fourth and was thrown out by Francisco Alvarez attempting to steal second.

Pete Alonso opened the sixth inning with a homer against Jose Ruiz that extended the Mets lead to 3-0. The blast was his second in three games — his ninth-inning homer Thursday in Milwaukee saved the Mets season.

The lefty Sanchez lasted five innings and allowed two earned runs on five hits and a walk with five strikeouts.

Severino got stampeded in the sixth, throwing a 99-mph fastball that Harper crushed for a two-run homer to pull the Phillies within 3-2. The next batter, Castellanos, jumped on a sweeper up in the strike zone for a 425-foot homer to left-center that tied the game.

Nimmo’s homer came with two outs in the seventh, after Orion Kerkering had struck out Lindor and Vientos to begin the frame.

Jose Butto plunked Realmuto in the seventh and allowed a two-out single to Rojas before Diaz entered to strike out Schwarber.