Arizona was within one out of its fourth straight win before J.J. Putz blew his second save of the season.
Leading 4-3 in the ninth inning Wednesday night, Putz got two outs after Bryce Harper's leadoff double. The Diamondbacks closer then gave up a game-ending homer to Ian Desmond that gave the Nationals a 5-4 win.
"It won't be the last time it happens to us this year," Diamondbacks manager Kirk Gibson said. "We're playing good baseball. We just have to shake this one off and come back tomorrow."
The Diamondbacks had no answer for rookie Bryce Harper, who helped the Nationals in the field, at the plate and on the bases.
He had three hits and made a tumbling barehanded catch as well as scoring his first run.
In the sixth inning, Harper fell down in the right-center gap while attempting to catch a fly ball off Jason Kubel's bat. As he landed in the grass, Harper secured the ball in his ungloved right hand.
"I was wondering if anybody saw...