Mets pitcher Noah Syndergaard delivers during the first inning against the...

Mets pitcher Noah Syndergaard delivers during the first inning against the Reds at Citi Field on Thursday. Credit: Jim McIsaac

Noah Syndergaard went from mystery to tying history Thursday afternoon at Citi Field. He ended his string of baffling poor outings by throwing a shutout and homering in a 1-0 victory over the Reds.

According to the Mets, that feat has happened only seven times since 1932, most recently on June 17, 1983, when Dodgers righthander Bob Welch did it against the Reds.

The 1-0 shutout and homer, Mickey Callaway pointed out, is rarer than the 23 perfect games thrown in the big leagues.

Syndergaard (2-3) did it in grand style on the mound and at the plate. He allowed four hits and struck out 10 in his third career complete game and second shutout. His earned run average is still elevated at 5.02, but it’s down from 6.35 before the start.

In the third inning, the lefthanded-hitting Syndergaard blasted an opposite-field, 407-foot homer to left for his second homer this season and sixth in his career, tying Tom Seaver and one shy of Dwight Gooden’s club record for pitchers.

It was one of only four hits allowed by Reds starter Tyler Mahle (0-3).

“I got that one pretty good,’’ Syndergaard said.

“That was a hard-hit ball and very deep,’’ Callaway said. “It looked like a righty hit it, a power-hitting righty.’’

Pete Alonso, who fits that profile, was watching from the dugout.

“He was unbelievable,’’ Alonso said. “He was a one-man wrecking crew out there today . . . He absolutely demolished that ball out there. That’s really special.’’

Syndergaard’s pitching performance was a major departure from his recent ledger: He had allowed at least four runs in five of his six starts.

Talk of it being cold and the ball feeling like an ice cube melted away after this start by Syndergaard, which allowed the tired — and ineffective — bullpen to take the day off.

“There’s no doubt, it’s the work,’’ Callaway said. “And it’s not just the work. It’s the work that he’s put in his whole career that allows him to do something like that . . . He used his curveball really well. He was able to do [his] fastball when he wanted to . . . He went out there and did what we need him to do. We don’t know how we get through the rest of the game if he doesn’t complete it, so great job by him.’’

Syndergaard sounded more relieved than overjoyed.

“I felt like I was pretty close to rock bottom,’’ he said of his past starts. “It’s kind of an adapt-or-die kind of situation. It feels like a huge weight has been lifted off my shoulders.’’

He said his success on the mound came from “just repeatability in my delivery, in tune with my mechanics. Kind of trusted that. Just keeping things simple and trusting every pitch, trying to win every pitch.’’

The Mets already have tied a franchise single-season record with four home runs by pitchers. Jacob deGrom and Zack Wheeler have hit one each. Callaway said they take hitting very seriously.

“I was out there early about four, five days ago. All of them were going [opposite field], like deep-deep,” he said. “So they get out there, they take pride in it. They know that they can help themselves win a game when they’re up. They take it more seriously than most.’’

As MLB potentially moves toward the universal designated hitter, Syndergaard is not in favor of adding it to the National League.

“I’m really not. I like hitting,’’ he said. “On the other side, a lot of the opposing pitchers don’t really know how to swing the bat all that well, so I just see it as an opportunity to get a couple of extra strikeouts throughout the game.’’

If the DH arrives, the opportunity to see these solo performances by pitchers will end.

“Today,’’ catcher Wilson Ramos said, “he did everything on his own.”

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