Jacob deGrom’s 10-strikeout performance against the Nationals on Saturday evaporated into an attempt to explain what went wrong in the Mets’ 3-1 loss at Citi Field.

“I’m not really sure what happened,’’ said deGrom, who notched his 12th career double-digit strikeout game after getting 13 against the Marlins in his last start.

DeGrom, who pitched 5 2⁄3 innings, started with three strikeouts in the first inning. He walked Bryce Harper for the first of what would become a career-high six walks for the righthander, including three (one intentional) to Harper.

“I felt like early on I had control of pretty much everything,’’ deGrom said. “And then I was having a hard time getting the ball down. I felt like I had good command early and kind of lost it late. Walking six guys, you’re behind on a bunch of counts. Just didn’t throw the ball how I wanted to late. Didn’t really make pitches when I needed to.’’

DeGrom, who allowed eight hits, said he did not feel any lingering effects from the stiff neck that pushed his start back a day.

He said of his outing: “It’s definitely strange. My goal is to go out there and put up zeros. I wasn’t able to do it. They had some hard-hit balls off me. Just wasn’t very good late.’’

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The Nationals scored their first run off deGrom in the fourth. After Harper drew a leadoff walk and Ryan Zimmerman struck out, Daniel Murphy singled to put runners on first and third. DeGrom walked Anthony Rendon and Matt Wie ters singled to drive in Harper.

The Nationals scored two more runs in the fifth. With one out, consecutive doubles by Adam Eaton and Trea Turner made it 2-0. DeGrom intentionally walked Harper and Zimmerman singled in Turner.

The inning was prolonged when Rendon walked, but deGrom prevented further damage by getting Wieters on a pop to short.

Terry Collins was pointed in his appraisal of deGrom, who is 0-1 with a 2.55 ERA in four starts and has struck out 32 in 24 2⁄3 innings.

“He didn’t hit his spots,’’ the manager said. “I mean, this guy struck out [10] guys. We checked with him every inning; he felt good, things were smooth, going along. They got a good lineup, you can’t make mistakes. Obviously, he didn’t make the pitches he needed to make.

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“He missed with his slider quite a bit tonight. He got some swings on some changeups [but] he missed the sliders on the outside corner, pitch after pitch. You know you got to get those to get outs. ’’

Catcher Rene Rivera said of deGrom: “I think overall he did a good job. That lineup, you minimize the damage [allowing] only three runs. They got Harper, Murphy, Zimmerman. You don’t want to deliver the ball down the middle of the plate.’’

Asked if the paucity of runs by the Mets is putting pressure on the starters, Rivera said: “I think it puts pressure on everybody. You want to do your best out there . . . We’re going to turn this around. It’s just a matter of time. One day we’re going to explode.’’