WASHINGTON — David Peterson has rehabilitated his season over the past few weeks with hardly flashy, but effective performances.

On Thursday, he walked the bases loaded in the first inning, escaping unscathed, albeit with an elevated pitch count.

As the starting pitcher, he got the Mets through the fifth with only one run allowed against the Nationals, reversing a recent trend:

In five appearances this season as a bulk reliever, he’s pitched to a 2.25 ERA.

In five as a starter before Thursday, he had an 8.10 ERA.

Peterson’s best start since opening weekend helped the Mets win 2-1 to split their four-game series at Nationals Park and snap a two-game skid.

“He’s a good pitcher, that is where you start,” manager Carlos Mendoza said. “He’s a real good pitcher that went through struggles, but we believe in him. Now it’s time for him to continue to do that and he will. He’s a big part of this team and we are going to continue to use him.”

The lefty had a brutal start against these Nationals on April 29, allowing seven earned runs, but since then has recovered, pitching to a 2.50 ERA over four appearances following an opener.

The Mets have needed him, especially with Clay Holmes’ deployment to the injured list over the last week with a fractured right fibula.

“I feel like I have been there for a couple of outings now,” Peterson said when asked if he felt as if he’s reverted to the pitcher who earned an All-Star appearance last season.

Huascar Brazobán, Brooks Raley, Luke Weaver and Devin Williams each pitched a scoreless inning behind Peterson.

Next stop for the Mets is a three-game series beginning Friday in Miami.

Peterson started his day with two fast outs before walking Andrés Chaparro, CJ Abrams and Dylan Crews to load the bases.

Peterson, who threw 28 pitches in the inning, struck out Daylen Lile to escape.

“There’s a couple of things I was feeling, but I was a little quick [in the first inning],” Peterson said. “I just tried to kind of settle in, slow myself down and get in my motion and then continue to try to get ahead with the first pitch and go from there.”

Bo Bichette’s fielding error to begin the second was overcome as Keibert Ruiz was nailed attempting to steal second, completing a strikeout/throw-out double play to end the inning.

Bichette stroked a two-run single in the third against Cade Cavalli to give the Mets their only scoring.

MJ Melendez got hit by a pitch and Luis Torrens and Carson Benge each singled to load the bases before Bichette delivered with a shot through the middle.

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It continued a hot stretch for Bichette, who was 7-for-19 (.368) with three homers over his previous four games.

He finished with six RBIs in the four-game series.

The Nats pulled to within 2-1 in the fifth on Chaparro’s RBI groundout.

Ruiz singled and James Wood doubled to put runners on second and third with nobody out.

After Chaparro brought in the run, Mark Vientos’ diving stop and scamper to first on Abrams’ smash prevented the Nats from tying the game.

“I felt like we were on the attack,” said Peterson, whose outing matched his longest since April 13. “I felt like we had a good game plan and just tried to fill up the zone and stay on the attack, be aggressive all day.”

The Mets loaded the bases with nobody out in the ninth, but couldn’t add insurance.

Melendez struck out, Torrens was retired on a line drive and Carson Benge flied out.

A.J. Ewing broke in the wrong direction on Lile’s fly ball leading off the bottom of the ninth and couldn’t secure the ball in his glove upon recovering.

Lile reached second and advanced to third on a groundout. Williams kept the tying run stranded by striking out José Tena before retiring Ruiz.

“That’s a good team over there,” Williams said. “We were able to split with them and come away with two [wins] and we have got to keep stacking good days.”