Bryce Harper slides home to put the Nationals up 3-2 in the fourth inning. Harper is 3 for 4 hitting in the seventh spot, scoring two runs with an RBI. (John McDonnell/The Washington Post)

As Ian Desmond strode to the plate for the Washington Nationals in the bottom of the ninth on Wednesday night against the Arizona Diamondbacks, Bryce Harper stood at second base. The 19-year old kid dubbed the “Chosen One,” with the funky hair, the moonshot-producing swing, the bravado and the rocket arm has been in the major leagues only five days — but dazzling ones at that.

Desmond — a holdover from the Montreal Expos days, a third-round pick in the 2004 draft who has known no other organization in his eight years of professional baseball — had a chance to show the key piece of the team’s hopeful future his first big league victory. And with one swing, he smashed a fastball from Diamondbacks closer J.J. Putz into the left-field bullpen, a two-run walkoff home run for a 5-4 win in front of an announced crowd of 16,274 at Nationals Park.

The team’s five-game losing streak was over. The Nationals (15-9) maintained their hold on first place in the National League East. Their franchise shortstop and their future outfield star featured prominently in a dramatic win. And the kid went 3 for 4 with two doubles, an RBI and two runs scored, injecting life into a struggling offense that lacked its two best hitters.

“Here’s a 19-year-old kid that’s getting the bat out,” Manager Davey Johnson said. “It’s infectious.”

Day Five of Harper’s major league career allowed him to add more accomplishments in his brief résumé. His first extra-base hit of the night, smashed to right-center field in the fourth inning, missed being his first big league home run by the width of the white “National League” lettering on the electronic scoreboard.

“I’m just trying to come in here and play my game hard,” Harper said. “I’m just trying to bring some fire to the table and play the game that I’ve known how to play my whole life. So I play with that fire and that passion, just trying to bust my butt every single day.”

But as talented and full of promise as Harper may be, however, the majors are still a new place for him. After an infield single in the second, Harper bit on Diamondbacks starter Joe Saunders’s first move to first base and was caught stealing easily to end the inning.

In the sixth inning, Harper took a circuitous route to a fly ball hit to deep center field by Jason Kubel. The twilight earlier had made it tough to see. Harper settled under the ball, then crouched to the ground, awkwardly using two hands to catch the ball, his bare right hand cradling the ball under his glove. “I was wondering if anybody saw that,” he deadpanned after the game.

Yet those plays seem almost excusable in light of Harper’s hustle and zeal. The kid will learn.

In the fourth, Harper showed his grit after falling behind 0-2 against Saunders and waiting out four pickoff attempts of Danny Espinosa at first. Saunders offered a 3-2 fastball and Harper countered with a furious swing. He thumped the ball to deep right-center field, a towering shot that, maybe for a second, he thought was as home run. The ball bounced off the electronic scoreboard. After he reached second standing with an RBI double that tied the score at 2, Harper pumped his fist and slapped his hands twice. Fans stood, cheered and captured the moment with their cameras.

Harper would again display his aggressive style a batter later. Second baseman Aaron Hill couldn’t cleanly field Ramos’s sharp grounder, and as Harper rounded third, Hill recovered and fired home to catcher Miguel Montero. The throw was on target and in time. All Montero had to do was apply a quick tag.

But Harper, sliding feet first, had his hands and arms out, bracing for contact. And as he slid into Montero, he seemed to coincidentally jar the ball loose. Home plate umpire Bill Welke called Harper safe and Montero was charged with an error for losing control of the ball. With his helmet off as he ran back into the dugout, and with the Nationals leading 3-2, the wind blew Harper’s wild hair onto his forehead.

“I was trying to make something happen at the plate and was going hard,” he said.

Nationals starter Edwin Jackson, however, couldn’t hold the lead. In the sixth inning, the Diamondbacks tied the game on a RBI double by Paul Goldschmidt and took a 4-3 lead on a run-scoring single to left by Ryan Roberts. Jackson pitched six innings on the night, allowing four earned runs on eight hits, with two walks and three strikeouts.

But Harper hadn’t given up yet. He led off the bottom of the ninth with another double to deep right-center field, only a few feet to the left of his previous double. Behind him, Ramos and pinch hitter Rick Ankiel struck out. Then came Desmond. “I was up there, I was calm, just looking for a pitch up in the zone,” he said.

He found one, and drove it to left field. Harper crossed home plate and pumped his fists. Desmond did the same as he rounded the bases. Just as Desmond had done his part, so did the kid.

“He brings a lot of energy to the team,” Desmond said of Harper. “That might be what we needed.”