WASHINGTON — Woof.

It was all set up Thursday for the Mets to win a second straight series rubber game — an accomplishment for a team that has floundered for most of the last month — but somehow this underachieving bunch managed to flatline.

Pitching, both starting and relief, faltered.

Bats that earlier looked ready for a breakout afternoon fell silent.

Lowly or not, the last-place Nationals kept fighting while the team allegedly competing for a postseason berth stopped.

Sean Manaea gets pulled during the Mets’ loss to the Nationals on Aug. 21. IMAGN IMAGES via Reuters Connect

The Mets took a three-run lead into the fifth inning then cratered, surrendering nine unanswered in a 9-3 loss at Nationals Park that sliced their lead on Cincinnati to a half-game for the NL’s third and final wildcard spot.

The Mets’ deficit on Philadelphia for first place in the NL East swelled to seven games.

So much for the small step forward the Mets took last weekend by winning two of three games against the Mariners.

Jacob Young singles during the Nationals’ win against the Nationals on Aug. 21. Brad Mills-Imagn Images

They now head to Atlanta for a rematch against a team that feasted against Mets pitching in winning two of three games last week at Citi Field.

Sean Manaea was close to untouchable early, but got sloppy in the middle innings and departed in the fifth.

The left-hander allowed four earned runs on three hits and one walk with two hit batters over 4 ²/₃ innings and was removed after 91 pitches.

Sean Manaea throws a pitch during the Mets’ Aug. 21 loss to the Nationals. IMAGN IMAGES via Reuters Connect

Francisco Lindor homered leading off the game, setting a franchise record in the process.

The blast was his eighth leadoff homer this season, which eclipsed the mark he shared with Curtis Granderson (who twice hit seven).

Starling Marte smashed a homer in the third that extended the Mets’ lead to 2-0.

It was the seventh homer this season for Marte and third in his past seven games.

The Mets loaded the bases in the fourth against MacKenzie Gore and received a sacrifice fly from Hayden Senger for the rookie catcher’s first career RBI.

Brett Baty and Cedric Mullins each singled in the inning, sandwiched around a walk to Tyrone Taylor — who was in the lineup with Brandon Nimmo sidelined by a stiff neck.

Sean Manaea reacts after throwing a pitch during the Mets’ Aug. 21 loss to the Nationals. IMAGN IMAGES via Reuters Connect

Manaea surrendered a run in the fourth on Dylan Crews’ RBI groundout.

The Nationals had threatened after CJ Abrams reached on a strikeout/wild pitch to open the inning, followed by Andres Chaparro’s single.

Manaea plunked Riley Adams to load the bases, but then retired the next three batters, with the run allowed.

Pete Alonso reacts after striking out during the Mets’ Aug. 21 loss. IMAGN IMAGES via Reuters Connect

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Gore was removed in the fifth after walking Marte and Pete Alonso successively.

But Jackson Rutledge retired Mark Vientos and struck out Baty.

Manaea surrendered three runs in the fifth after Alonso’s throw to second was late trying to nail lead runner Brady House.

Mackenzie Gore reacts after allowing a home run to Starling Marte on Aug. 21. IMAGN IMAGES via Reuters Connect

Francisco Lindor rounds the bases during the Mets’ Aug. 21 loss to the Nationals. IMAGN IMAGES via Reuters Connect

Jacob Young reached first on the grounder, and after Abrams got drilled to load the bases, Paul DeJong’s sacrifice fly sliced the Mets’ lead to 3-2.

Chaparro walked to reload the bases before Tyler Rogers entered and allowed a single to Adams that brought in the tying and go-ahead runs.

Young’s RBI single in the sixth against Rogers extended the Nationals’ lead to 5-3.

Daylen Lile singled and stole second in the inning before Young delivered with a two-out shot off Mullins’ outstretched glove while diving.

Ryne Stanek dumped kerosene on the fire by surrendering four runs in the eighth.

House stroked an RBI single before James Wood delivered the haymaker with a three-run homer.