PHILADELPHIA — In a departure from the script of the past few days, the Mets jumped out to a lead that provided legitimate hope they could survive a game, if not quite turn the proverbial corner.

But what started as promise clarified as a tease. And for the Mets, what started as a downturn sure is looking like a collapse.

Carlos Mendoza’s group blew a four-run lead en route to getting swept, saving their most painful loss of the series for last in a 6-4, knife-twisting defeat Thursday.

And now the 76-71 Mets — who were 45-24 and led the NL East by 5 ½ games on June 12 and already have waved goodbye to the division — are in real danger of blowing the season.

As recently as Friday they were tied for the second wild-card spot and held a four-game lead over the Giants and a six-game lead over the Reds.

Six consecutive losses later, they cling to a 1 ½-game wild-card edge over San Francisco and Cincinnati.

“I’m not going to say that I’m worried, but we don’t have too much time,” Mendoza said with 15 games remaining, the next one against Jacob deGrom. “Teams are right behind us. … The way we look at it is: We still control the situation.”

Reed Garett reacts during the Mets’ 6-4 Sept. 11 loss to the Phillies. Corey Sipkin for the NY Post

They controlled the situation Thursday, too, in grabbing a four-run bulge six batters into the game. Jesús Luzardo and Jhoan Duran then combined to pitch a perfect 8 ²/₃ innings the rest of the way while the Phillies’ offense went to work.

“We’re playing a couple good teams … we just got to get going,” said Jeff McNeil, whose inexperience in center field may have cost them. “Luckily for right now, we’re in a playoff spot.”

Virtually everything — from yet another too-brief outing from their starting pitcher to defensive miscues to relief issues to an offense that disappeared — went wrong in a loss that might not be the season’s worst but, considering the circumstances and timing, ranks among the season’s most painful.

Jeff McNeil chases a Phillies hit during the Mets’ Sept. 11 loss. Corey Sipkin for the NY Post

There was extra pain because there had been belief. Run-scoring hits from Mark Vientos, Brandon Nimmo and Starling Marte created the first-inning lead.

After scoring a combined three runs in four games against Hunter Greene, Aaron Nola, Ranger Suárez and Christopher Sánchez, the Mets topped that before recording a second out against Luzardo.

“We didn’t do anything after that,” Mendoza said, accurately, after Luzardo and Duran (who struck out the side) sat down the final 25 batters, a stretch in which Mets frustration was evident.

Vientos slammed his bat and helmet after a strikeout. McNeil tossed his equipment away after his second strikeout of the night.

Mark Vientos reacts after striking out during the Mets’ Sept. 11 loss to the Phillies. Corey Sipkin for the NY Post

With the offense MIA, soon enough the lead was gone.

“Gotta play better; simple as that,” said David Peterson, who allowed three runs in five innings before watching his bullpen and defense contribute to a three-run bottom of the sixth that several Mets will want to forget.

McNeil did not acquit himself well. Nick Castellanos sent a gapper to left-center that the center fielder cut off quickly enough to have a play at second, but McNeil’s throw was off line and allowed Castellanos to slide in safely.

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Otto Kemp then cracked a drive to deep center on which McNeil originally broke in before retreating too late, the game-tying double sailing over his head.

“Just made a bad read on it,” said McNeil, a veteran but playing his 37th career game in center. “Saw the pitch, it was down and away, and he lunged for it. Nine times out of 10 on a swing like that, the ball’s going to be in front of you.”

Reed Garrett did not acquit himself well, either, giving up those hits plus a two-out single to Harrison Bader, which provided the go-ahead run.

Bryce Harper singles during the Phillies’ Sept. 11 win against the Mets. Corey Sipkin for the NY Post

And Brooks Raley did not acquit himself well, walking Kyle Schwarber before Bryce Harper hammered a one-hopper down the first base line to Pete Alonso, who did not acquit himself well. Alonso knocked the ball down but could not corral it, becoming an RBI single that created the final score.

Several around the Mets reiterated talking points that are not questioned: They are fighting; they are talented; they control how the rest of this season unfolds.

But clearly alarms are being sounded in a somber clubhouse, Mendoza declining to reveal whether he addressed the group after the latest crusher.

“I’m responsible. I’m the manager,” Mendoza said. “It’s my job to get these guys going, and I will.”