NEW YORK (AP) — Noah Syndergaard came out zinging it, firing a 98 mph fastball past Bryce Harper to end the first inning.

Tuning up for a postseason start, the 23-year-old rookie was sharp. The rest of the New York Mets, they still have some work to do.

Harper wound up denting the Mets' playoff position, hitting a tiebreaking home run in the eighth inning that gave Washington a 3-1 win Saturday in the opener of a day-night doubleheader.

The Mets lost their fourth in a row and fell one game behind the Los Angeles Dodgers in the race for home-field advantage in their NL Division Series matchup next week. Each team has two games left.

"No preference at all," Syndergaard said. "We prefer to pitch at home and open at home, just because we have the best fans in baseball. But we're going to get to the Dodgers anywhere we are. I'm excited for things to come."

Syndergaard struck out 10, giving up just two hits in seven innings in what amounted to a playoff tuneup on a misty, chilly day. The doubleheader was forced by a rainout Friday night.

The 23-year-old went 9-7 with a 3.24 ERA in 24 starts in his first big league season. He fanned 166 and walked 31 in 150 innings.

"I feel like I ended the regular season on a high note today. I feel like I learned a lot this season, and now it's time to transfer what I learned during the season in the playoffs," Syndergaard said.

"When I came up here and made my debut, I was more of a thrower. Now I think I've advanced myself to more of a pitcher, and I'm really excited at the strides I've made this season," he said.

Harper's 42nd homer was a two-run, two-out shot off Addison Reed (3-3). Acquired late in the season from Arizona, Reed hadn't allowed a run in his first 15 appearances for the Mets.

Harper tied Colorado's Nolan Arenado for the NL lead in homers. The long drive to right field came right after second baseman Daniel Murphy bobbled a potential double-play grounder, and gave Harper 99 RBIs.

The top MVP candidate began the day with a two-point lead over Miami's Dee Gordon in the NL batting race. As some fans chanted "Papelbon" — a reference to his recent dugout brawl with now-suspended teammate Jonathan Papelbon — the Nationals star was hit on the inside of the left shin by a pitch from Syndergaard in the sixth.

Harper briefly went down on the dirt and Mets catcher Travis d'Arnaud patted him on the back. Harper was checked by a trainer and stayed in the game.

Rafael Martin (2-0) got one out as the Nationals ended a six-game skid against the Mets. Felipe Rivero worked the ninth for his second save.

Before the game, Mets manager Terry Collins talked about his team trying to regain its "edge." Instead, the club fell to 5-6 in their last 11.

Mets star David Wright struck out three times. When he fanned to finish the fifth, he flung his bat and slammed down his helmet so hard that it nearly bounced to the dugout.

Gio Gonzalez shut out the Mets on three hits for six innings. New York broke its 23-inning scoreless streak in the seventh on an RBI single by Juan Lagares off Matt Grace.

Clint Robinson homered in the Nationals seventh.

Collins planned to mix-and-match his lineups in the doubleheader. He started Lucas Duda and Murphy against Gonzalez in the opener, hoping the lefty vs. lefty matchups would help them prepare to face Dodgers ace left-hander Clayton Kershaw.

Jonathon Niese, moving from the rotation to the bullpen for the playoffs, threw a scoreless ninth for the Mets. One of his outs came when Robinson tried to stretch a single into a double, and another came on a pickoff that led to a caught stealing.

TRAINER'S ROOM

Mets: LHP Steven Matz got an injection for his sore back and "hopefully that helps him a lot," Collins said. The 24-year-old rookie is 4-0. He missed two months because of a partial muscle tear on his left side earlier this season, and hasn't pitched in a game since Sept. 24. The Mets would like him to start against the Dodgers — he could get a tuneup next week in the instructional league.

UP NEXT

Nationals: RHP Max Scherzer (13-12, 2.91 ERA) starts the second game of the doubleheader. He'll try to finish over .500 in his first season with Washington.

Mets: RHP Matt Harvey (13-7, 2.80) is 5-0 in his last 10 starts. He's thrown 183 1-3 innings this season — there had been a lot of talk lately that 180 was his target. Harper is 0 for 20 with 11 strikeouts vs. Harvey.