WASHINGTON -- Bryce Harper showed why his manager put him in the No. 3 spot for his fifth career game.
The 19-year-old phenom hit a tiebreaking RBI double in the sixth inning, lifting the Washington Nationals to a 2-1 victory over the Arizona Diamondbacks on Thursday night.
Ross Detwiler (3-1) did his part as well. The left-hander allowed one run and 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 inning.
Ian Kennedy (3-1) lost for the first time in 13 starts, snapping a personal nine-game win streak.
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.
Washington manager Davey Johnson moved Harper up four spots in the lineup a day after he went 3 for 4 with two doubles, and the rookie delivered. He is now 6 for 16 in his five games, with four doubles and three RBIs.
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.
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.
Both starters posted a strong outing, with Kennedy yielding four hits in seven innings with four strikeouts and one walk.
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.
The Diamondbacks tied it in the sixth inning. Willie Bloomquist walked with one out and scored on Aaron Hill's double which eluded a diving Werth in right-center.
NOTES: Nationals 1B Adam LaRoche, the team's regular cleanup hitter, was scratched because of a sore right side muscle. Chad Tracy took over for LaRoche at first base. ... 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. ... 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.