WASHINGTON - Jonathon Niese had spent much of his season working through soreness in his left arm, doing whatever it took to keep pitching. He even adjusted his throwing mechanics to lessen the strain.

He found a way to thrive.

So, after a nearly three-week stint on the disabled list, which allowed him the rest needed to restore his health, Niese returned with his arsenal intact. He reverted to his previous mechanics, since he was free from the soreness that stemmed from a strained left shoulder.

Instead, Niese has stumbled to his roughest stretch of the season, which continued Wednesday night in the Mets' 7-1 loss to the Nationals.

"I'm at the point now that my stuff's back,'' Niese said. "My arm feels great. I just need to locate with that better stuff.''

Perhaps a fix is within reach, though against the Nationals, Niese (5-8) hardly stood a chance.

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Danny Espinosa's three-run homer in the sixth inning knocked the Mets down for good. It came after Adam LaRoche's two-run shot in the first, a towering blast that crashed off the facing of the upper deck in rightfield. It was his first of two homers.

Niese allowed a season-high six runs in six innings before he was pulled by Terry Collins, who is convinced the lefthander is past his physical issues.

"We've got to keep getting him out there, keep getting him work,'' Collins said.

Not that Niese got much help. The Mets' offense has slumped in the second half and the culprits have been conspicuous. David Wright was 1-for-4 and hit into a double play. He's hitting just .194 since the All-Star break. Curtis Granderson bunted for a hit to end an 0-for-18 skid. But after going 1-for-4, he's hitting .161 in the second half.

Doug Fister (11-3) held the Mets' comatose lineup to one unearned run in 71/3 innings. He struck out seven while walking none.

"He works real quick, throws a lot of strikes, and doesn't walk anybody,'' Collins said.

The Nationals (61-51) extended their lead in the NL East to eight games over the Mets (54-60), who will send thriving rookie Jacob deGrom to the mound this afternoon in hopes of capturing the rubber match of a three-game set.

This season, the Mets have won just twice in eight games against their division rivals.

Niese had been the Mets' most reliable starter in the first half, even as the soreness in his arm worsened with every start. He landed on the disabled list just before the All-Star break, hoping that the time off would get him back on track.

His return has been brutal. Niese fell to 0-4 with a 5.76 ERA in the second half. He hasn't won since June 28, a stretch of five consecutive decisions. And against the Nationals, Niese allowed more than one homer in a game for the first time since April 12.

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The Mets fell behind 3-0 in the first, when Anthony Rendon doubled ahead of Jayson Werth, whose bloop single put the Nationals on the board. LaRoche followed with his 14th homer, hammering an 88-mph fastball that sailed over the heart of the plate.

Espinosa put the game away with a three-run homer in the sixth, crushing a changeup for his seventh homer of the season.

Niese's fastball averaged 90 mph, an indication that his stuff has returned since his injury. But he paid for a failure to locate his pitches.

Said Niese: "I'm making a lot of mistakes now.''