Jacob deGrom of the Mets pitches in the first inning against...

Jacob deGrom of the Mets pitches in the first inning against Atlanta at Truist Park on Thursday in Atlanta. Credit: Getty Images/Kevin C. Cox

ATLANTA — When the aces’ duel became a battle of the bullpens, the Mets lost to Atlanta, 3-2, on Thursday night in what represented a huge swing game in the greater context of the quest for an NL East title.

Had the Mets won, they would have escaped town with the same 5 1/2-game lead over second-place Atlanta that they arrived with, which would have been awfully comfortable-seeming given the rest of their schedule. They also would’ve guaranteed a victory in the season series — more important this year, since it is the division tiebreaker if the teams finish the regular season with the same record.

Instead, the Mets’ NL East lead is at 3 1/2 heading into a stretch of four games in less than 48 hours in Philadelphia. They are still positioned well, to be sure, just not as well as they were before losing three of four at Truist Park this week.

This was the Mets’ first series loss against a division opponent in 17 chances this season. The record is 18 (1970 Reds).

“Definitely disappointing,” Jacob deGrom said. “I wanted to come in here and win all four games. Once they won a couple, then the goal was for the split. And we weren’t able to get it. Disappointed with it, but we gotta go play tomorrow.”

Brandon Nimmo said: “I’m not disappointed. Maybe some other guys are. Obviously you would love to every game, but it’s just not realistic in baseball. It’s not realistic in 162 games. We gotta try to put this behind us as best as we can.”

Michael Harris II, the first Atlanta batter to face someone other than deGrom, had the seventh-inning hit off Seth Lugo that stood as the eventual game-winner. It was a ground ball that bounced past the mound and a reaching Lugo, sneaking through the middle of the infield and a defensive shift, into center. Nimmo ran in from left-center, fielded and fired, sort of, the cutoff man Darin Ruf, who was near the mound.

Vaughn Grissom — off with the two-out, full-count pitch — obeyed orders from third-base coach Ron Washington, who was waving him home all the way. He scored from first, beating Ruf’s relay.

Nimmo said he was comfortable with his approach to the play, explaining that he slowed down before fielding the ball because “the last thing I want to do is … miss it, then give them the extra base — or home — without a play.” 

“By the time it got to me it was barely rolling, so I knew there was probably going to be a play at the plate,” Nimmo said. “That ball was hit in the right spot. Kudos to him for putting it in play. Good things happen when you put the ball in play. The only thing I wish I would’ve done maybe is let it rip to home plate.”

The Mets (76-43) stranded the potential tying run at second in the eighth and ninth.

For most of the night, deGrom (6 2/3 innings, three runs) and Max Fried (seven innings, two runs) acted as equals.

DeGrom contributed his longest outing of four this season by innings and pitches (95, a jump of 19 from his previous high), boasting a fastball that was even faster than normal (99.9 mph) and a slider that he threw more frequently than any other offering. He had nine strikeouts, zero walks and three runs allowed.

“It felt good,” deGrom said of going long, relatively speaking. “It felt like I still had it. I actually felt like my slider got better later in the game.”

Manager Buck Showalter said: “The level that he pitches at, people expect perfection every time he cocks his arm — and he’s close to it. We just didn’t score any runs.”

Blame Fried, who held the Mets to four hits, struck out six and walked none. Mark Canha hit a two-run homer — to end an eight-pitch at-bat — in the fifth. Fried helped Atlanta (73-47) elude trouble after a runners-on-the-corners, no-outs situation in the third.

That was Fried’s first start in nearly two weeks because he had been on the concussion IL.  

“You feel good that he’s healthy and didn’t have any problems with a concussion,” Showalter said. “But wish he’d waited another day.”