NEW YORK (AP) - With their once-promising season quickly slipping away, the New York Mets needed R.A. Dickey to stop the slide Tuesday night.

Instead, it was Dickey's winning streak that came to an end.

Adam LaRoche hit a tiebreaking homer and the Washington Nationals handed the knuckleballer his first loss in three months, beating the sinking Mets 5-2 behind Gio Gonzalez's seven crisp innings.

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``The results were poor,' Dickey said. ``I wish I would have done better.'

Gonzalez (13-5) gave up only two singles in a rematch of All-Star pitchers, five days after New York hit him hard to back Dickey during his 13th win. This time, it went the other way.

Dickey (13-2) allowed five consecutive hits in the sixth, snapping his 11-game winning streak, as the Mets lost their fifth straight and 11th in 12 games. Their lone victory since July 7 was Dickey's 9-5 win over Gonzalez last Thursday in Washington.

In a mini shake-up before the game, New York demoted slumping right fielder Lucas Duda to the minors.

``Now we've got to see who is willing to grind it out,' manager Terry Collins said. ``We've got to figure out something. We're not a bad team, but we're playing bad right now.'

Ryan Zimmerman kept up his tear at the plate with three hits and the Nationals won their fourth straight to improve the NL's best record to 57-39. They are 8-3 against the Mets this season, winning all four series so far, and will go for a three-game sweep Wednesday at noon with ace Stephen Strasburg on the mound against rookie Jeremy Hefner.

``When you're not playing good baseball against teams like this, they exploit you pretty quickly,' Mets third baseman David Wright said. ``This stretch is not going to define us over the course of the year.'

Sean Burnett got three outs in the eighth and Tyler Clippard fanned two in a perfect ninth, finishing the three-hitter for his 17th save. Gonzalez struck out four, walked two and did not allow an earned run in his longest outing since May 27.

``It was a masterpiece,' Nationals manager Davey Johnson said. ``He had something to prove.'

Jordany Valdespin set a Mets record with his fifth pinch-hit homer of the season. The rookie connected off Ryan Mattheus leading off the eighth, breaking a mark he previously shared with Mark Carreon (1989) and Danny Heep (1983).

``It's great,' Valdespin said. ``I hope it's the beginning of a long career for me here. I hope I can keep it going.'

Dickey was 11-0 in his previous 16 starts since getting roughed up in a 14-6 loss April 18 on a rainy day in Atlanta. Trying to become the first 14-game winner in the majors, he was unable to stop New York's nasty slide. He yielded five runs - four earned - and eight hits in six innings.

The 37-year-old right-hander tossed an inning of relief Saturday against the Dodgers to help an overworked bullpen, only two days after throwing 111 pitches over 7 1-3 innings in Washington. He has a 5.36 ERA in his last seven outings.

``I had a good knuckleball,' Dickey said. ``I should have pitched into the eighth inning. I felt too good for that to happen.'

Dickey whiffed star rookie Bryce Harper all three times on knuckleballs ranging from 66 to 81 mph and was in command until the sixth, when he gave up five straight hits with two outs.

Michael Morse singled and LaRoche drove a low knuckler into the upper deck in right, snapping a 1-all tie with his 17th homer. Danny Espinosa singled, Roger Bernadina beat out a slow dribbler toward first and Jesus Flores hit an RBI single. Left fielder Jason Bay's wide throw to the plate got away for an error that allowed Bernadina to score, making it 5-1.

Espinosa's throwing error at shortstop led to an unearned run in the second. Ronny Cedeno hit an RBI single to right with two outs, giving New York its first lead in 42 innings since last Thursday, but was thrown out by Harper trying to stretch it into a double.

Gonzalez retired his next 11 batters and did not permit another hit until Ike Davis' two-out single in the seventh.

A switch-hitter, Espinosa batted right-handed against Dickey to get a better look at the knuckleball. The first time up, Espinosa struck out swinging. But in the fourth, his two-out double tied it 1-all after Zimmerman opened the inning with a bloop double.

NOTES: Harper is 0-for-10 against Dickey with six strikeouts. ... New York released RHP Miguel Batista, who was designated for assignment Sunday. ... Mets rookie OF Kirk Nieuwenhuis has a new-look haircut after his golden locks were trimmed. Asked if the change had more to do with the hot weather or his falling batting average, Nieuwenhuis said: ``Both.' ... Desperate for bullpen help, New York signed RHP Craig Hansen to a minor league contract that runs through next season. The 28-year-old righty, who went to nearby St. John's, was drafted 26th overall by Boston in 2005 but has not pitched professionally since 2010 in Pittsburgh's minor league system. He is 4-9 with a 6.34 ERA in 95 big league games, with his last appearance coming for the Pirates in 2009. He will report to the Mets' complex in Florida to begin a throwing program.