Pitcher Trevor Cahill took advantage of a Washington lineup without any teeth as he pitched the Arizona Diamondbacks to a 5-1 victory over the Nationals on Tuesday.
Cahill allowed just one run in 7 1/3 innings against one of the worst offenses in baseball this season, which led to Washington (14-9) falling to their fifth successive loss.
John McDonald and Miguel Montero delivered RBIs in the fifth and sixth innings respectively, and Arizona (13-11) added two more in the seventh to take a 4-0 lead and hold on for their fourth victory in five games.
Washington's highly touted prospect Bryce Harper, making just his third start in the majors and first at home, finished 0-for-3 at the plate.
Harper's call-up was intended to help add some batting power to the Nationals, who have been carried by a strong pitching staff this season.
Starter Jordan Zimmerman was charged with four runs in 6 1/3 innings and took the defeat.