WASHINGTON — A couple of defensive slip-ups and a questionable relief option sealed the Amazin’s first loss of the year, a 4-2 defeat to the Nationals on Sunday at Nationals Park.

The Nationals erased the Mets’ 2-1 lead in the eighth inning after right-hander Trevor Williams came in to relieve southpaw Chasen Shreve with no outs and a runner on first in a one-run game.

“Trevor was going to pitch today and Chasen was going to pitch today,” said Mets manager Buck Showalter. “Can’t let them go 4-5-6-7 days into the season (without pitching). They pitched well, both of them. I know it’s a result-oriented business, but if you look at the type of balls they gave up … I’m happy with it.”

Williams got into trouble right away. He allowed a single to his first batter, Maikel Franco, to put his inherited runner, Dee-Strange Gordan, on third base and Franco on first with no outs.

Then came the defensive miscues, as the ball kept finding first baseman Pete Alonso.

“I didn’t make the throw,” Alonso said of his wide throw to second base that loaded the bases for the Nationals, setting up their eventual win. “I didn’t make the play.”

After Franco’s single, Nationals shortstop Lucius Fox placed a sacrifice bunt toward the charging Alonso, who hesitated on his throw to home plate. The speedy Strange-Gordan beat Alonso’s underhand flip to the dish, and the Nats tied the game. Showalter said Alonso had a tough time getting it out of his glove.

“I thought I made the right play,” Alonso said. “If I were to throw it, I feel like it’d be too quick and too close of a distance. I wanted to secure the baseball and make a good flip to (catcher Tomas Nido). I did that, it’s just that he’s fast.

“I thought everything was clean. Dee-Gordan is one of the fastest players in the league. Sometimes, even though it’s well-defended, you have to tip your hat to the baserunner.”

Showalter echoed Alonso on his read of that safety squeeze, adding: “You got a guy that runs not good, but real well. Not sure what somebody else could’ve done.”

Moments later on a Cesar Hernandez ground ball, Alonso threw the ball wide to shortstop Francisco Lindor, and instead of turning a double play, everybody was safe.

Nelson Cruz soon sealed the Mets’ loss after notching a two-run single against Williams, ripping a ball to left field.

“On the double play ball, I just missed the throw,” Alonso said. “I don’t know why. My feet were set. I fielded the ground ball cleanly. There was no rush. I feel like I was calm, cool and collected. I just missed the throw, and as a result that kind of put the team in a really bad hole.”

The Mets depart for Philly on Sunday having won three of their first four games of the year. With the rebuilding Nationals not a real threat for the Mets in the otherwise competitive NL East division, the Amazin’s leave Washington having done what they came to do: win the series.

But Alonso still shouldered full responsibility for Sunday’s loss.

“It’s good to win the first series of the year, but again, dropping this one late kind of stinks, especially for me,” Alonso said. “I didn’t want to do that. When I go out there, I don’t want to let anyone down, especially not anyone in the locker room. After not making the play there, I let the team down and as a result we lost the game.”

As for why Showalter stuck to his word and made sure to get both Chasen and Williams into the series finale, the skipper said some of his pitchers, though he didn’t name them, were unavailable on Sunday. Entering the game, both Seth Lugo and Adam Ottavino had pitched in two of the team’s first three games. Trevor May at one point in the seventh inning was warming up in the bullpen, but he never came into the game.

Edwin Diaz is on the bereavement list after his grandfather passed away during Friday night’s game. He is eligible to return on Tuesday, which is when the Mets expect him back from Puerto Rico. Showalter will manage the bullpen again on Monday in the series opener against the Phillies without his closer.

“It’s too early in the season to be throwing guys three out of four days,” Showalter said. “I said the whole offseason, with the lockout and everything, I’m going to be careful.”

Carlos Carraso’s terrific season debut – 5.2 innings and just two hits allowed – had set the stage for Mets bats to do some damage. So, for Showalter skipper, the reason the Mets lost on Sunday was simple: “We just didn’t score. We scored two runs today. We had some opportunities that we didn’t cash in on.” The Mets, after amassing 34 hits in their first three games thanks to timely hitting, went 1-for-8 with runners in scoring position to close the series.

Francisco Lindor collected his first home run of the season and Mark Canha went 3-for-4 with an RBI. But Starling Marte had the day off, and Brandon Nimmo went 0-for-5, with three of those luckless at-bats coming against right-hander Erick Fedde, as other regular starters struggled to get going at the plate.

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