WASHINGTON -- Only five games into his major league career, Bryce Harper is already an essential part of the Washington Nationals' lineup.

The 19-year-old phenom moved into the No. 3 spot and responded with a tiebreaking RBI double in the sixth inning, lifting the Nationals to a 2-1 victory over the Arizona Diamondbacks on Thursday night.

"I want to win every single game," said Harper, who hit seventh in his first four games. "I'm going to try to come up as clutch as I can in those kind of situations. I love those situations."

The new spot in the heart of Washington's order didn't seem to faze the No. 1 overall pick in the 2010 amateur draft.

"If I'm hitting two, three, four, five, six, seven, eight or nine it doesn't really matter," he said. "It doesn't change the way I'm going to go up there and play."

Harper seized the spotlight again, but Ross Detwiler (3-1) was equally impressive. 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.

"I was really impressed with Det," Washington manager Davey Johnson said. "He was my star of the game."

Even Detwiler, however, deferred to his young teammate.

"The star? I got it over Bryce?" he said. "I felt good. I really wish I could have finished that seventh. My location was just off."

Ian Kennedy (3-1) lost for the first time in 13 starts, snapping a personal nine-game win streak. He allowed four hits, struck out four and walked one in seven innings.

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.

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.

"He's a baseball player," teammate Jayson Werth said. "When you're a baseball player, you can be 15 or you can be 50. If you know how to play the game you can play."

Harper also had an interesting night on the basepaths.

Harper hit a weak grounder back to Kennedy in the first, but the pitcher's throw was wide and he 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. Harper said later he wasn't sure if he touched it or not.

"I thought I might have graced over it a little bit," he said. "I'm just trying to busy my butt down the line and trying to make them hurry, and they called me out."

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.

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.

"Play like that's infectious," Johnson said. "We have that makeup on our ballclub. We play hard. He takes it to another level."

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. Johnson said he wasn't sure how long LaRoche would be out, but said it might be two to three days.
• 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.