WASHINGTON — Train wreck.

Disaster struck the Mets in Tuesday’s ninth inning, the likes of which may resonate until the season’s conclusion, as manager Mickey Callaway’s bunch attempts to remain relevant.

A six-run lead in the ninth inning should be a slam dunk, but the Mets hit the rim. Kurt Suzuki smashed a three-run walk-off homer against Edwin Diaz, completing the epic meltdown at Nationals Park and sending shock waves through the visitors’ dugout.

It was the first collapse of that magnitude in Mets history. Prior to Tuesday, they were 806-0 when leading by six runs in the ninth inning or later, according to Elias Sports Bureau.

“Definitely hard to wrap your head around,” Brandon Nimmo said after the Mets’ 11-10 loss to the Nationals. “Coming back from [six] runs down in the bottom of the ninth against guys throwing 99 mph, I don’t have words for that.”

The Mets led 10-4 entering the inning, but got only one out. Paul Sewald surrendered four hits in the inning and departed with two runs already in and two runners on base. Luis Avilan was summoned and surrendered a ground-ball single to Juan Soto before Diaz entered and allowed a two-run double to Ryan Zimmerman. The next batter, Suzuki, cleared the left-field fence. The Mets fell five games behind the Cubs in the race for the NL’s second wild card.

“When I saw [Suzuki’s] ball, I just said that this game is over,” Diaz said. “We battled the entire game, the offense battled, [Jacob] deGrom pitched a great game, but at that point I just knew we had lost the game.”

According to ESPN, teams leading by six or more runs entering the bottom of the ninth inning were 274-0 before the Mets’ meltdown.

Seth Lugo had retired all three batters he faced in the eighth with the Mets ahead 5-4. But after the Mets scored five runs in the ninth, highlighted by Nimmo and Pete Alonso homers, Callaway inserted Sewald. The goal was to rest Lugo for potential duty in Wednesday’s series finale.

“You have got a six-run lead, major league pitchers have got to be able to hold that,” Callaway said. “It’s a little tougher [loss] than some others, but we’ll get through it because we have a resilient bunch in there.”

McNeil had homered leading off the eighth and drove in two additional runs in the ninth, but was among the Mets left wondering what had just hit him.

“We are chasing the Nationals, chasing the Cubs, and a game like that we have got to win,” McNeil said.

The owners of the past three NL Cy Young awards — both of whom are among those contending for this season’s prize — Jacob deGrom and Max Scherzer each had their hiccups on this night.

Soto clubbed deGrom’s 100th pitch of the game for a two-run homer in the eighth that pulled the Nationals within 5-4. DeGrom was promptly removed, after allowing four earned runs on eight hits with three walks.

Scherzer’s downfall was a fourth inning in which the Mets sent eight batters to the plate and scored four runs. The right-hander allowed only one other base runner in his eight innings pitched.

DeGrom had a second straight start of allowing four earned runs over seven innings as he battles Scherzer, Hyun-Jin Ryu and Mike Soroka, among others, in the Cy Young race.

Suzuki’s RBI single in the sixth pulled the Nationals within 4-2, but Matt Adams’ brain cramp helped derail a potential big inning. Adams retreated to first on Suzuki’s shot to center that hit off the fence and got only as far as second base. That left runners on first and second for Gerardo Parra, who hit into an inning-ending double play.

Joe Panik’s two-run homer was the Mets’ big hit in a four-run explosion against Scherzer in the fourth. In the inning, the Mets sent eight batters to the plate and collected five hits.

Wilson Ramos doubled in a run to tie it 1-1, extending his career-longest hitting streak to 26 games. With the hit he tied David Wright for the second-longest hitting streak in Mets history. Moises Alou holds the club record with a 30-game streak.

“That’s a tough loss,” deGrom said. “To have a pretty nice lead there and end up losing that game, that’s a tough loss. I am sure guys are going to be upset until it’s time to play [Wednesday].”

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