R.A. Dickey was looking for a late-season win over baseball's top team to improve his Cy Young Award chances.

In the end, one pitch really hurt him.

Bryce Harper had his first four-hit game and pinch-hitter Tyler Moore launched a go-ahead homer off Dickey in the seventh inning to send the Washington Nationals past the punchless New York Mets 5-3 on Tuesday night.

"That's the best lineup I've faced. They're just so functional," Dickey said. "They've got guys who can run, they've got great energy, they've got old guys who have good approaches, they've got young guys that go first to third on you. They've got power. Every one of them can hit a homer, just about. There's not really a lot of breathing room."

Dickey (18-5) mostly pitched out of trouble for seven solid innings. But he gave up a two-run shot to Moore that made it 3-2 in the seventh and was unable to tie Nationals lefty Gio Gonzalez for the major league lead in wins.

Gonzalez beat New York 5-1 in the series opener Monday.

Not surprisingly, Dickey got little help from his teammates at the plate. The fading Mets, losers of five in a row and seven of eight, set a club record by failing to score more than three runs for the 12th consecutive home game. The last time they managed four runs at Citi Field was Aug. 12 in a 6-5 win over Atlanta.

New York has dropped 21 of its last 25 in Queens and is 0-5 on a six-game homestand against Atlanta and Washington. The Nationals improved to 13-4 against the Mets this season.

"I had a mediocre knuckleball and had to pitch with it," said Dickey, who had won three consecutive starts. "At this point in the season, you want to give the fans something. We've got a shot at doing something, maybe."

After entering 0 for 10 with six strikeouts against Dickey, Harper doubled and singled twice off the All-Star pitcher. The talented rookie added an RBI single in the ninth against Josh Edgin to become the first teenager with four hits in a major league game since Andruw Jones did it for Atlanta on Sept. 22, 1996.

"Now he realizes he can hit a knuckleball," Nationals manager Davey Johnson said.

Leadoff batter Jayson Werth reached base all five times for the NL East leaders, who have won seven straight and 11 of 12 at Citi Field.

Harper credited Werth's fine at-bats in front of him for helping him solve Dickey. Werth is 13 for 27 (.481) with two homers, three doubles and five walks against Dickey — attributing his success to all the Wiffle Ball he played as a kid.

"That's kind of how I approach it," Werth said. "I feel comfortable."

Tom Gorzelanny (4-2) worked a hitless sixth in relief of Jordan Zimmermann, who threw 104 pitches in five innings. Christian Garcia struck out three of his four batters and Tyler Clippard allowed a solo homer to pinch-hitter Scott Hairston in the ninth before securing his 31st save in 34 attempts.

Kurt Suzuki also had an RBI single in the ninth for Washington (88-54).

After going an absurd 110 straight innings at home without scoring more than one run, the Mets finally put up a crooked number in the fifth.

Ruben Tejada singled for the third time and scored easily from first when Daniel Murphy's slicing double took an odd carom off the retaining wall in medium left field. Murphy clapped his hands at second base, Dickey cheered from the dugout and then David Wright grounded an RBI single under the glove of a diving Ian Desmond at shortstop.

"I felt really good with R.A. on the mound. You're thinking he's going to get us to the ninth inning and we're going to give ourselves a chance here," Mets manager Terry Collins said. "It didn't happen."

Suddenly given a 2-1 lead, Dickey soon gave it back. Suzuki singled with one out in the seventh and Moore drove the next pitch to left for his ninth home run in 138 at-bats this season.

Harper shortened his big swing against Dickey, hitting the ball on the ground all four times. After fouling off a bunt attempt, the 19-year-old slugger chopped a double inside third base to set up Ryan Zimmerman's sacrifice fly in the first.

"Just trying to see something up or see a pitch I can square up," Harper said. "I've tried everything against that guy."

In a tradition that began after the terror attacks of Sept. 11, 2001, the Mets wore caps commemorating the NYPD, FDNY and other first responders during batting practice and the national anthem.

Hours earlier, the team held a meet and greet with players and Tuesday's Children, a non-profit family service dedicated to helping individuals affected by 9/11 and terrorism worldwide.

NOTES: Johnson said he met New York quarterbacks Eli Manning and Mark Sanchez at a charity function near the World Trade Center site Tuesday morning. "It's just a great cause," the manager said. "It was nice to be there." ... Nationals LHP John Lannan, from nearby Long Beach, will make his first start at Citi Field since April 2010 when he faces Mets rookie Matt Harvey in the series finale Wednesday night. ... New York RHP Jenrry Mejia is probably lined up for three starts the rest of the way, Collins said. The Mets also plan to give RHP Jeurys Familia his first big league start during the final week of the season.