NEW YORK -- Center fielder Bryce Harper launched a tie-breaking, two-run homer with two outs in the eighth inning Saturday afternoon as the Washington Nationals beat the New York Mets 3-1 in the opener of a day-night doubleheader at blustery Citi Field.

With the win, the Nationals (82-78) locked up their fourth straight winning season while damaging the National League East-winning Mets' hopes of earning home field advantage in the NL Division Series against the Los Angeles Dodgers.

New York is 89-71 and a game behind the Dodgers entering play Saturday night. If the Mets and Dodgers finish tied, the Mets get home field based on winning the season series 4-3.

Harper, a season-long target of Mets fans, absorbed chants of "Jonathan Papelbon!" -- in "honor" of the Nationals closer who is suspended for the season after attacking Harper in the dugout last Sunday -- and heard some cheers when he was plunked on the left knee by a pitch from right-hander Noah Syndergaard in the sixth.

But Harper got the last laugh in the eighth. With one out, second baseman Anthony Rendon walked against right-hander Addison Reed (3-3). Third baseman Yunel Escobar then hit a grounder behind second base, but shortstop Ruben Tejada initially mishandled it and could only force Rendon at second.

Harper homered into the second deck in right field three pitches later. The homer was his 42nd of the season, tying him with Colorado Rockies third baseman Nolan Arenado for the NL lead. He is one RBI shy of 100.

Right-hander Rafael Martin (2-0) got the final out of the seventh. Right-hander Casey Janssen threw a perfect eighth and right-hander Felipe Rivero threw a perfect ninth for his second save.

First baseman Clint Robinson was 2-for-4 with a seventh-inning solo homer for the Nationals.

For the Mets, center fielder Juan Lagares had a game-tying RBI single in the seventh.

Nationals left-hander Gio Gonzalez exited with the lead after allowing three runs and three walks while striking out seven over six shutout innings. He finished the season 11-8 with a 3.79 ERA in 31 starts.

Syndergaard allowed one run on two hits and one walk while striking out 10 over seven innings. Syndergaard, who is expected to start Game 2 of the NL Division Series next Saturday, ended his rookie season 9-7 with a 3.24 ERA in 24 starts.

NOTES: Mets club ambassador Rusty Staub is resting comfortably in an Ireland hospital after suffering a medical emergency during a flight from Ireland to New York. ESPNNewYork.com reported Staub had a heart attack. Staub, 71, played nine seasons with the Mets. ... Mets LHP Steven Matz (back) received an injection Friday. Manager Terry Collins said Matz could still start Game 4 of the National League Division Series on Oct. 13 if he is able to throw 90-100 pitches in an instructional league game on Wednesday. ... MLB.com reported Friday that the Nationals have fired two front office members: Director of international scouting Bill Singer and advance scout Bob Johnson. ... The doubleheader Saturday was the 26th for the Nationals since the franchise relocated from Montreal in 2005.