The Arizona Diamondbacks wasted one of Ian Kennedy's best performances of the season.

Kennedy pitched seven crisp innings, but Bryce Harper hit a tiebreaking RBI double to lead the Washington Nationals to a 2-1 victory on Thursday night.

Kennedy (3-1) gave up a season-low four hits, struck out four and walked one while matching his longest outing of the year. It was his first loss in 13 starts, snapping a personal nine-game win streak.

"It's always really tough when we lose these tough games," he said. "It's frustrating, I hate losing, it doesn't matter how it is."

The NL East-leading Nationals took two of three from Arizona and have won five of their six series this season. Henry Rodriguez pitched the ninth for his sixth save.

Arizona has lost two in a row after winning four of five.

"I think we played pretty good baseball," Arizona manager Kirk Gibson said. "Sometimes you come out ahead, and sometimes you don't. The guys played good, they played hard, Both teams pitched well, played well, but they got more guys in than we did tonight. We feel good about ourselves and we'll come back tomorrow."

Aaron Hill hit a tying RBI double in the sixth, but that was it for the Diamondbacks against Ross Detwiler (3-1). The left-hander allowed three hits over 6 1-3 innings in his longest outing this season. He retired the first eight batters he faced and didn't allow a hit until the fifth.

It was Harper, though, who stole the show for the Nationals, starting before the game when manager Davey Johnson moved the 19-year-old up four spots to No. 3 in the lineup.

"He has really quick hands, which is not surprising, but one of the things I've noticed is he's played really hard," Kennedy said. "That's all you can really ask for with somebody at his, I guess at his status, where he's at."

Harper also had an interesting night on the basepaths.

He hit a weak grounder back to Kennedy in the first, but the pitcher's throw was wide and the No. 1 overall pick in the 2010 amateur draft took second on what appeared to be an error.

The Diamondbacks decided to appeal and Harper was called out for failing to touch first base.

"It was just a bad throw," Kennedy said. "I just didn't set my feet. Thankfully, he didn't touch first base."

Ian Desmond, who hit a game-ending homer in Wednesday's 5-4 victory, started the Washington sixth with a double. He moved up on Steve Lombardozzi's sacrifice and scored on Harper's opposite-field double down the left-field line.

Jayson Werth followed with a hard grounder to third baseman Ryan Roberts. Harper waited, then broke for third on Roberts' throw. Harper just beat the return throw from first baseman Paul Goldschmidt, but he was stranded at third when Chad Tracy flied out to right.

The Nationals scratched out a run in the second for a 1-0 lead. Werth led off with a double and Chad Tracy followed with a walk. They advanced on Kennedy's one-out balk, and Werth scored on Rick Ankiel's bouncer to first.

Kennedy said he wasn't sure what led to the balk call.

"I didn't feel like I did anything different from the other I don't know how many pitches I threw that inning before that," Kennedy said. "That's what was really confusing."

NOTES: Kennedy's previous loss was on August 18, 2011, in Philadelphia. He gave up three earned runs in three innings, then was removed after a rain delay. ... Harper was the youngest player to bat third in a major league lineup since Andruw Jones did for Atlanta in 1996, according to STATS LLC.