That brought up Upton, who had been benched in four of the Braves' previous five games amid a season-long slump that had seen him strike out 62 times in 158 at-bats.
The high-priced free agent, however, this time delivered a solid hit for the game-winner, allowing the National League East-leading Braves to even their three-game series.
Jordan Walden (2-1) notched the decision with a 1-2-3 top of the 10th after starter Tim Hudson had held the Nationals to one unearned run over 7 1/3 innings.
Nationals starter Gio Gonzalez also pitched well in a no-decision, yielding one run on three hits while striking out seven through seven frames.
Ian Desmond provided Washington's only RBI.
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