Michael Conforto and Pete Alonso of the Mets celebrate after...

Michael Conforto and Pete Alonso of the Mets celebrate after scoring on an RBI double by Wilson Ramos in the first inning against the Nationals at Nationals Park on Saturday in Washington. Credit: Getty Images/Patrick McDermott

WASHINGTON — Up by seven runs with three outs to go, Seth Lugo found that his simple task of retiring three batters, starting with the bottom of the Nationals’ lineup, wasn’t all that simple.

It was a blowout, the stands were mostly empty and Lugo was lacking the requisite adrenaline. In trying to pump himself up, he lost control of his pitches, walked two batters, hit another and allowed four runs (all unearned because of Amed Rosario’s error).

The Mets hung on to win, 11-8, on Saturday to give themselves a shot at a season-opening sweep of the Nationals, but not before manager Mickey Callaway had to pull Lugo in favor of closer Edwin Diaz, who threw one pitch for his second save in as many games.

“It felt like [expletive],” Lugo said.

That dampened an otherwise well-rounded win for the Mets, who had big days from Jeff McNeil (4-for-5, homer shy of the cycle) and Pete Alonso (3-for-4, two RBI doubles), plus noteworthy contributions from reserves J.D. Davis (go-ahead two-run single in the eighth in his team debut) and Dominic Smith (1-for-2 with two RBIs in only two innings of action).

Callaway said he stuck with Lugo so long because he knew the righthander would be unavailable anyway after warming up twice and throwing one pitch to get one out in the eighth inning. As the bottom of the ninth dragged on — walk, error, strikeout, walk, HBP, strikeout — and the righthander’s pitch count climbed, “I was hoping to get that last batter,” Lugo said.

Then Ryan Zimmerman roped a three-run double to left, turning a gimme win into a sudden save situation.

“I still had faith that he was going to only give up that one run,” Callaway said. “Then he got ahead of Zimmerman, he was making good pitches to him, couple of chases, then Zimmerman got him. Save situation comes up, we have to get him out of there.”

Early, the Mets gave Callaway a lot to like, specifically with their offensive approach. Callaway and hitting coach Chili Davis are emphasizing using the whole field — especially the vast opposite field when a batter is facing a defensive shift — and merely putting the ball in play, not trying to hit it out.

Of the Mets’ seven run-scoring hits, five went to center or the opposite field. Highlighting that at-least-for-a-day trend: Wilson Ramos’ two-run double off the wall in right during a three-run first inning, Davis’ hit in the eighth and both of Alonso’s doubles.

The first-inning rally started with the lefthanded-hitting Michael Conforto grounding a single through the left side.

“And everybody else followed suit,” Callaway said. “We kept the train moving.”

Both of Alonso’s doubles were smoked — the first a one-hopper to the centerfield wall, the second off the scoreboard in right-centerfield. That was enough to earn strong praise from Noah Syndergaard, who referred to Alonso as “The Polar Bear,” a nickname assigned to him by third-base coach Gary DiSarcina.

“The Polar Bear, trying to knock down outfield walls,” said Syndergaard, who allowed four runs in six innings in his season debut. “Alonso’s got tremendous talent. It’s only begun. He’s scratching the surface.”

Alsonso said he learned from facing Stephen Strasburg (also four runs, six innings) twice during spring training how to approach him in a game that counts. That game plan paid off, with Alonso picking up two hits in his first two at-bats against the righthander.

“I made my own scouting report,” Alonso said. “I made a plan going into today, I looked back on my notes, executed my plan and stayed within myself and stayed under control, which was the main focus today.”

The game was close until the Mets scored seven runs in the last two innings. Then it got close again as Lugo wobbled.

“[Mets hitters] did a great job of grinding out every at-bat, making it tough on them,” Callaway said. “And it ended up we needed those last two innings’ worth of runs.”

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