(Reuters) - 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.

"That's the best he's thrown for us," Arizona manager Kirk Gibson told reporters of his starter. "Much more into the zone, commanding the zone early and often, and when you can do that, you can get early swings."

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.

"I feel like we have hitters who can hit, but were are not as aggressive as I would like to see," said Nationals manager Davey Johnson.

Starter Jordan Zimmerman was charged with four runs in 6 1/3 innings and took the defeat. He got into trouble in the seventh, where he trailed 2-0, and was relieved with the bases loaded and charged with the two ensuing runs.

Ian Desmond drove in the Nationals' run in the eighth with a single to centre field before Justin Upton singled in a run in the ninth for Arizona.

Upton finished 2-for-4 with two RBIs and Paul Goldschmidt added three hits for Arizona.