Washington’s Laynce Nix slides into home plate in the fourth inning. Washington blew a 5-1 lead and a shot at a sweep of the Braves in Atlanta. (Paul Abell/AP)

Jordan Zimmermann did not flinch as he watched all he had done unravel. He leaned on the railing in front of the Washington Nationals’ dugout, both arms slung over the top, and tracked the ball off Martin Prado’s bat as it tore through the night sky and his team’s hopes for a sweep. Once it landed, Zimmermann stared out to the field, turned around and disappeared down the dugout steps.

Minutes earlier, Zimmermann had left one of the most dominating starts of this Nationals season, and his team was rolling to a the victory that would cap a defining road trip. When Prado’s seventh-inning grand slam off Sean Burnett cleared the fence, it stunned the Nationals and set the stage for a 6-5, 10-inning, walk-off loss to the Atlanta Braves and a dejected visitors’ clubhouse at Turner Field.

The game ended when Doug Slaten, pitching his third inning of relief, allowed a single to right by Brian McCann, scoring Prado from second base. Slaten, a lefty who’s usually a specialist, had previously thrown 62 / 3 innings all season. He had only pitched two innings in a game four times in his career, never more. With the bullpen ravaged from recent use and shortened by two relievers with questionable pedigrees, though, Manager Jim Riggleman stayed with Slaten.

Burnett’s implosion and Slaten’s bitter end obscured Zimmermann’s dominance. He walked two, allowed five hits and struck out 11 batters in 61 / 3 innings, the most since June 8, 2010, the day Stephen Strasburg climbed a major league mound for the first time and struck out 14.

“We easily could have walked away from here with a win,” Burnett said. “Zim threw such a good game. It makes me sick to think I blew a four-run lead.”

The Nationals led 5-1 entering the seventh inning, cruising toward a signpost victory. They could have woken up Friday morning with a record above .500 for the first time in 23 days, having swept the Braves and finished a multi-series trip with a winning record for the first time since May 2008.

Zimmermann went out for the seventh inning having thrown 90 pitches and retired the first hitter he faced. He walked Freddie Freeman on a 3-1 slider, which he would later call the pitch he regretted most, and yielded a bloop single to Alex Gonzalez, ending his night and bringing Burnett into the game.

Burnett jumped ahead of the first hitter he faced, Brooks Conrad, with two strikes, only to walk him after what Riggleman thought was an incorrect check-swing call. “That irritates the heck out of me,” he said.

Up came Prado with the bases loaded. Prado fell behind 1-2 and then launched into an epic at-bat. He survived six more pitches, including four two-strike foul balls, until the count ran full.

“I kept making good pitches,” Burnett said. “He kept fighting them off. The one slider I threw him was behind him. I don’t know how the hell he got to it.”

Fans tomahawk-chopped as Burnett steadied on the mound. He fired a sinker that stayed belt-high — “I finally made a mistake,” Burnett said. When Prado crushed it, the stands exploded. A comfortable lead had been erased with one swing, replaced by a tie score.

Riggleman had planned to use Burnett and Drew Storen to finish off the game. He did not want to use Todd Coffey, who had been used frequently, or Tyler Clippard, who also has been used and would be needed Friday night. “I had to have something for tomorrow,” he said.

Slaten navigated through the eighth and ninth unscathed despite two shots to the warning track, one of which bounded off the wall. He felt he had to save Storen, because Brian Broderick, a Rule 5 pick, and Henry Rodriguez, an out-of-options flamethrower who has been allergic to the strike zone, have proved unusable in crucial spots.

So Slaten went out for the 10th. He walked the leadoff batter, Prado. Three hitters later, after a sacrifice bunt and an intentional walk, McCann ripped a fastball into the right field corner. “Disappointing,” Slaten said.

It also wasted Zimmermann’s gem. Zimmermann loaded the bases in the first inning, then froze Eric Hinske with a 79-mph curve to end the inning. Zimmermann made one misstep the rest of the game until the seventh, a fastball over the plate in the third that Chipper Jones hit halfway to Macon. His fastballs zipped at 95 mph, and his slider darted across the plate and at hitter’s ankles. He became the eighth Nationals’ pitcher to strike out 11 batters in a game.

“I felt great all night,” Zimmermann said. “Everything was working. I kind of got in a groove there, and I felt awesome. I felt strong.”

The Nationals grabbed control when Danny Espinosa blasted his fourth home run of the season, a two-run shot off Derek Lowe just left of center field. The Nationals added two more runs in the fourth, and Ivan Rodriguez jacked a solo home run, his second of the year, in the fifth.

From there, the game suddenly unraveled. Zimmermann could only watch. As Burnett pitched to Prado, Zimmermann thought, “Let’s get a groundball, get out of this.” When the exact opposite happened, he swallowed hard and moved on.

“It’s baseball,” Zimmermann said. “Things like that are going to happen. We’ve just got to move forward and get them tomorrow.”