NEW YORK — A sign above the doorway at one end of the Mets’ clubhouse reads: Do Your Thing.

Sage advice for the defending NL champs down the stretch — no matter who is on the mound.

Curtis Granderson and James Loney came through with big hits to back Robert Gsellman, and New York got another gritty start from an unheralded rookie in its banged-up rotation to beat the Washington Nationals 3-1 on Saturday night.

“We’ve asked a lot of some young players and they have stepped up,” Mets manager Terry Collins said. “If you would have told me in April that I’m going to look out on the field and I’m going to have three players that were in our opening day lineup and four rookie pitchers to win a game, I’d tell you you’re out of your mind.”

Gsellman (2-1) pitched out of trouble for six innings, and inexperienced center fielder Michael Conforto made a diving catch that thwarted a potential Nationals rally in the seventh.

New York rebounded from consecutive losses and won for the 10th time in 14 games, moving within one game of St. Louis for the final NL playoff berth.

“We’re right there,” Loney said. “This is what we play for.”

Tanner Roark (14-8) allowed two runs over five innings and walked a career-high four — one intentional. He was lifted for a pinch hitter in the sixth, but Washington never broke through and had its four-game winning streak snapped.

Anthony Rendon hit a bases-loaded sacrifice fly in the first. Danny Espinosa struck out all four times up for the Nationals, who still have a 9 1/2-game lead in the NL East over the Mets.

“He’s swinging too hard when we need to put the ball in play,” Washington manager Dusty Baker said. “That’s something that we’re addressing and he’s got to keep working on it.”

Making his third major league appearance and second start, the long-haired, 23-year-old Gsellman was pumped up after wriggling out of a jam in the sixth.

With runners at second and third, he fanned Espinosa and got pinch hitter Ben Revere on an inning-ending groundout.

“Just take a deep breath and don’t get too ahead of yourself,” Gsellman said after his first career win as a starter. “Make a good pitch and see what they do with it.”

Collins said Gsellman had “about as good a sinker as I’ve seen from anybody.”

Jayson Werth singled with one out in the seventh and ex-Met Daniel Murphy hit a line drive to center field. Conforto, however, charged in and made a sensational play that was probably more acrobatic than wise.

“It definitely felt good to get that one,” Conforto said. “Puts us in a bad spot if it gets past me, for sure.”

Rookie left-hander Josh Smoker struck out NL MVP Bryce Harper on a breaking ball way out of the strike zone to end the inning.

Addison Reed tossed a 1-2-3 eighth and Jeurys Familia whiffed two in a perfect ninth for his major league-leading 45th save in 48 tries, extending his franchise record.

New York had lost four straight and seven of eight to the Nationals.

“It was kind of a nothing game when we didn’t come through all game,” Baker said after his team finished 0 for 7 with runners in scoring position.

On fireworks night at Citi Field, Granderson hit a two-run single in the third on the first pitch after Yoenis Cespedes was intentionally walked to load the bases with two outs.

Loney, also struggling in clutch situations and dropped to eighth in the lineup for the first time all season, added an RBI double with two outs in the sixth off Matt Belisle. Travis d’Arnaud scored easily from first base when Harper overthrew two cutoff men from the right-field corner.

BREAKING OUT

Granderson got his first RBIs since July 6 that did not come on a home run or sacrifice fly. He had been 0 for 22 with runners in scoring position dating to July 24, and 1 for 31 in those situations since July 7. … With his first extra-base hit since Aug. 1, Loney ended slumps of 0 for 14 and 1 for 20.

TRAINER’S ROOM

Nationals: Max Scherzer’s next start was pushed back a day to Monday to give the ace right-hander extra rest.

Mets: The team is leaning toward starting RHP Rafael Montero in place of RHP Jacob deGrom (forearm) on Tuesday night at Cincinnati, Collins said. To get him ready, Montero had his outing Saturday night for Double-A Binghamton cut to 29 pitches. … Injured 1B Lucas Duda (back) recently started swinging a bat again and the club has not ruled out a return late this season. … RHP Zack Wheeler has been shut down for the season. Wheeler had Tommy John surgery in March 2015 and was unable to make it back this year because of several setbacks in his rehabilitation — most recently a strained flexor muscle.

UP NEXT

Nationals: Rookie RHP Reynaldo Lopez made his major league debut July 19 and is 2-2 with a 5.33 ERA in five starts for Washington.

Mets: Weather permitting, rookie RHP Seth Lugo (2-2, 2.60 ERA) makes his fourth career start Sunday night and tries to win his third in a row.