Washington left-hander Ross Detwiler outdueled Chicago's Jeff Samardzija, and the NL East-leading Nationals clinched the franchise's first winning season since 2003 by edging the Chicago Cubs, 2-1, in the opener of a four-game set.
Detwiler (9-6) scattered four hits and three walks over seven scoreless innings, while an Adam LaRoche solo home run was among the seven hits given up by Samardzija (8-13) over his seven frames of work.
"He missed his spot with me and I put the bat on it," LaRoche said.
Jeff Beliveau replaced Samardzija to begin the eighth and Bryce Harper greeted him with a leadoff single before scoring on a Ryan Zimmerman double into the left-field corner.
The run proved to be the difference in the game after Tyler Clippard gave up an RBI base hit to Welington Castillo with two outs in the ninth, the Cubs' lone hit with a runner in scoring position.
Castillo drove in Anthony Rizzo, who had led off with a single and advanced to second base on defensive indifference. Tony Campana, the pinch runner, then moved into scoring position on a botched pickoff attempt, but Clippard struck out Josh Vitters to earn his 30th save.
"Anytime you can play first place teams, the games mean a lot and they're fun games," Samardzija said.
The Washington franchise last posted a winning record when the Montreal Expos went 83-79 nine years ago ... The Nationals, at 82-52, have recorded the most wins since the club moved to Washington in 2005 ... LaRoche homered in the second inning, a leadoff blast to right field ... Washington left fielder Michael Morse left Monday's game with right thumb soreness ... The Cubs began a 10-game trip and have now lost 14 of 15 on the road.