Gio Gonzalez leaves the mound after getting pulled in the fifth inning. (Toni L. Sandys/The Washington Post)

It’s early in the season, but if this competitive National League East race comes down to one game, the Washington Nationals may want to look back at three moments in Sunday’s 3-2 loss to the Atlanta Braves to see where, in an early June game against a division rival, it went wrong. A distracted pitcher missed covering first base by a step. An overeager rookie got thrown out trying to stretch a double into a triple. And, the team’s best player grounded into a double play with two runners in scoring position in a key late-game situation.

“It’s a tough loss,” Steve Lombardozzi said.

The Nationals have managed to stay in first place in the National League East, the only division in the NL in which all teams have winning records, despite a rash of injuries to vital players and thanks, in large part, to the strength of their starting pitchers. And while they maintained their hold on first place over the Miami Marlins by a sliver, losing close games because of small mistakes yields little joy.

The game started with unusual heroics, back-to-back home runs from bench player-turned-everyday-leadoff hitter Lombardozzi and teenage phenom Bryce Harper. And on the mound was one of the major league’s best pitchers so far this season, Gio Gonzalez, who had been on a recent string of stellar outings.

Spot Gonzalez a quick lead and all should seemingly go well against the Braves. Lombardozzi, a 23-year-old local from Columbia who had only 17 home runs in over 1,700 minor league at-bats, hit one on the third pitch he saw from starter Tommy Hanson. He hit the ball so hard it slammed off the back of the bullpen wall in right field for his first big league homer.

One pitch later, the 19-year-old Harper did him one better, crushing the ball into the second deck in right field. The crowd roared for two of the Nationals’ brightest young players. It was the first time rookies have hit back-to-back home runs to lead off a game in the modern major league era, according to the Elias Sports Bureau.

“As soon as they started getting up to high-five me, Harper hit his,” Lombardozzi said. “It was pretty funny.”

Then came those three crucial mistakes, starting in the fifth inning.

For much of Sunday’s first four innings, Gonzalez continued his mastery of his new surroundings. The Nationals shipped four of their best prospects to the Oakland Athletics for Gonzalez during the offseason because they saw potential in the former all-star, despite his reputation for inaccuracy. He has blossomed into one of the major leagues’ best pitchers, with a rising fastball, sharp curve and good location.

“His command was way off today,” Manager Davey Johnson said. “He really couldn’t get the ball in and he wasn’t using the inside of the plate. Just a rare occasion that he didn’t have pretty good command and today was one of them.”

Gonzalez showed signs of laboring more than normal, taking 66 pitches to work through four innings, falling behind and missing high. Then the blunder occurred. First baseman Adam LaRoche nabbed a ground ball hit by the speedy Michael Bourn deep in the hole, and Gonzalez ran to cover first but he did so with little urgency, and Bourn slid safely head first into the base, putting a runner on with no outs.

“I should have gone out there and exploded right off the mound and got that ball,” Gonzalez said. “Was lost a little bit in the stands but it’s no excuse. He’s just quick and you can’t take your eye off him for a second.”

Gonzalez, who has been so sharp much of the season, then unfurled two wild pitches, one of which sailed over the head of catcher Jesus Flores. An intentional walk to Nationals nemesis Dan Uggla and a walk to Matt Diaz loaded the bases for left-handed hitter Jason Heyward with two outs. Heyward slapped the second pitch he saw to left field to score two runs. Just like that, the lead was lost, in an inning that should have been over.

Gonzalez smacked the ball into his glove before leaving the mound. It was only the second time this season, and the first in 10 starts, that the left-hander failed to get through at least five innings.

The Nationals threatened thereafter but none more disconcertingly than in two instances. In the bottom of the fifth, Harper drilled a ball hard to right field, hitting the out-of-town scoreboard and missing another home run by only a few feet.

As Bourn and Heyward converged on the ball, it rattled along the right field wall. The relay throw from shortstop Andrelton Simmons to third baseman Martin Prado beat Harper by a few steps. The rookie stood up, bent over in disgust and slammed his helmet on the ground.

“He had to make a perfect throw, and he made a perfect throw,” Harper said. “Nine times out of 10, I don’t think he makes that throw.”

Added Johnson: “He’s just a young, aggressive player and I like to see it, but that’s a case where we’re in a situation I need you in scoring position.”

Lombardozzi and Harper would again give the Nationals a shot in the eighth inning, notching leadoff walks off Braves reliever Eric O’Flaherty. But Ryan Zimmerman, the cornerstone of the team but who has been mired in a long slump since returning from a shoulder injury, provided another frustrating end to a promising opportunity. He rolled over a slider, hitting it sharply at second baseman Uggla for a double play.

“I just stink right now,” Zimmerman said. “It’s frustrating.”

The Nationals had three chances to seize Sunday’s game and build a firmer hold on first place. Instead, those instances resulted in mistakes, and a loss.