Mets catcher Wilson Ramos connects for a first-inning grand slam against Nationals starting pitcher Jeremy Hellickson on Tuesday night. (Jonathan Newton/The Washington Post)

The double play came four innings too late. Washington Nationals starter Jeremy Hellickson trudged back to the mound with two outs in the fifth inning, and New York Mets second baseman Robinson Cano, who’d hit the ball, veered from the base path to the dugout. But the damage, seemingly irreparable with the Nationals’ recent offensive performance, had already been done.

The Nationals’ inability to convert a first-inning twin killing sparked a Mets rally that eventually doomed Washington in a 6-2 series-opening loss to New York, its 14th defeat in 19 games. The Nationals’ anemic offense followed up Los Angeles Dodgers starter Hyun-Jin Ryu carrying a no-hitter against them into the eighth Sunday by allowing Mets starter Noah Syndergaard to do the same into the sixth. By the end of the night, Washington had scored in just 10 of its previous 79 innings.

Manager Dave Martinez acknowledged “we just got to swing the bats better,” and he understood the offense shouldered a portion of the blame, but he placed the lion’s share on the defense Tuesday night.

“That first inning bit us,” Martinez said. “We’ve got to play clean baseball. We really do. Got to give us a chance. That was a different game [if] we get out of that inning.”

The key sequence came with a runner on first and one out in the first. Hellickson got Cano to ground a change-up to second baseman Brian Dozier for what looked like a perfect double play ball. On contact, that’s what Hellickson thought would happen, and Martinez said the team should turn it “10 out of 10 times.”

Dozier flipped to Wilmer Difo for the out at second, but the shortstop’s low and wide throw to first dragged Gerardo Parra, once a Gold Glove outfielder, off the base. “If he just makes a better throw, Robinson’s out by five feet,” Martinez said.

“He didn’t have to really throw the ball that hard.”

When asked if he thought so, too, Difo didn’t blink.

“I probably rushed [the throw] a little too much,” he said. “I didn’t realize that [Cano was] probably not as fast as I expected him to be.”

For a low-velocity, high-contact pitcher such as Hellickson, sound defense is crucial. He struggles to remain effective on already-thin margins without it. The defense has struggled behind everyone this season, but it seems as though Hellickson has suffered the miscues from the mound as often as anyone.

In this case, Hellickson compounded the mistake with a few of his own.

He walked Pete Alonso and allowed a single to Michael Conforto to bring up former National Wilson Ramos. The catcher delivered the fatal blow to his old club, lining an 81-mph change-up over the fence in left for a grand slam. Hellickson got the next batter to fly out, but the hole was dug.

The Nationals (16-25) have allowed mistakes to snowball this season. Difo noted that the offensive issues put more pressure on the defense.

“It feels that way right now, especially since the team seems to be struggling a little bit,” he said. “As a team, we’re fine, but those errors have been costing us big lately. What we need to try to do is take a step back, calm down and try to eliminate those errors. . . . Things will turn for us.”

Even with at least 27 outs left, even with Syndergaard appearing more vulnerable than usual with his career highs in ERA (5.14) and walks plus hits per innings pitched (1.33), the early deficit still looked steep. On Sunday, Ryu’s no-hit bid got the attention, but Washington left Los Angeles with bigger problems. The Nationals ended that series by scoring in one of their previous 27 innings.

Against the Nationals, Syndergaard looked vintage. He mowed through the lineup, striking out four in the first three innings and limiting contact mostly to soft groundballs. He made two mistakes — hitting Adam Eaton to lead off the game, walking Anthony Rendon to start the second — but never let those runners advance past first.

By the end of the fifth, the Nationals had more grand slams allowed (two) than hits (one) in their past 14 innings.

The offense sparked in the sixth. Difo atoned for his earlier errant throw by lining a change-up into center to break up Syndergaard’s no-hit bid. Three batters later, after the Nationals hit into a double play, Victor Robles smashed his seventh home run deep into the left field seats. It was the outfielder’s third homer of the season off Syndergaard. It wound up mattering little.

In the last three innings, the Nationals mustered one hit. In the ninth, for a Nationals team proven adept at hindsight, it was easy to look back and think how this might have gone differently had the defense turned that double play in the first, had Hellickson made better pitches after the miscue, had the Nationals not grounded into a double play a few innings later.

In his postgame news conference, Martinez still saw the defense as the problem. If the team converted the double play, then suddenly the Robles homer delivers a 2-1 lead. The manager repeated a familiar message and, though he might have felt it, he didn’t sugarcoat it by saying the boys played hard.

“We do extra drills. We take groundballs every day,” Martinez said. “Sometimes, when a team’s struggling, you try to do a lot more. Just play the game. Really, all you’ve got to do is play the game. You’ve done it a million times. That’s a double play ball, 10 out of 10 times. Just play the game. Don’t make it hard on yourself. Just do the little things.”