The Nationals mob Denard Span after he hit the go-ahead RBI in the bottom of the ninth to beat the Atlanta Braves, 6-5. (Katherine Frey/The Washington Post)

Late Tuesday afternoon, in the top of the ninth inning, Denard Span stood in center field at Nationals Park and thought, “It’s not going to be too good for us.” Two defensive lapses had gifted the Atlanta Braves an extra run. Craig Kimbrel loomed. The team they couldn’t beat would hand a two-run lead to the closer they couldn’t touch. Back in the clubhouse, sitting at his locker, Tyler Clippard was still cursing himself. The Washington Nationals’ improbable rush to contention had reached the cliff’s edge.

“The numbers were probably pretty well stacked against us, to be honest,” shortstop Ian Desmond said later. “But they have been for a while now.”

Minutes later, Span dashed away from onrushing teammates in the middle of the diamond, celebrating the Nationals’ most improbable victory of an improbable late-season surge. The Nationals became the first to ever score three runs off Kimbrel in a 6-5 walk-off win, the last two sprinting home after Span’s bouncer up the middle skipped through the legs of shortstop Andrelton Simmons, one of the league’s most sure-handed fielders.

As the Nationals mobbed Span, Tanner Roark had just arrived at the park. “It got me so amped up, I wanted to start the game right then and there, five minutes later,” Roark said. Roark kept calm, and then at night dominated the second game of the doubleheader. The soft-spoken 26-year-old right-hander from Illinois continued his turn from call-up to sensation in the Nationals’ 4-0 victory. Roark threw seven scoreless innings, outdueling veteran Freddy Garcia, lowering his ERA to 1.08 in 412 / 3 career innings and upping his record to 7-0.

A day after the Washington Navy Yard shooting, the horror that unfolded four blocks from Nationals Park, baseball resumed in the District. The Nationals gave anyone who wanted it a pop-up pitching idol, a moment to remember and a doubleheader sweep they could sink their teeth into.

The Post Sports Live crew tries to see meaning in the Nationals late season run even though the team is a longshot to make the playoffs. (Post Sports Live/The Washington Post)

“I really, really wanted it bad, just to fire up people that are watching,” said Game 1 starter Dan Haren, who allowed one run over six innings. “Just to make a few people feel good. I wanted, I really wanted the game bad. I’m glad I did well, and it was even cooler to see how we came back.”

The Nationals scored their first run in Game 2 when Steve Lombardozzi smoked a single into right field that scored Bryce Harper from second base. Ryan Zimmerman started a monster insurance rally in the eighth when he blasted Jordan Walden’s fastball five rows over the high wall in center. Harper, Desmond and Adam LaRoche added two more runs with three straight hits.

In the fifth, Span ripped a single to left and extended his hitting streak to 28 games, the longest in the majors since Dan Uggla’s 33-game tear in 2011. But the most remarkable day came from Roark.

“Tanner today was one of the best performances I’ve seen all year, out of any of our guys,” LaRoche said. “Fun to play behind, quick game, quick innings.”

“It’s just not changing anything,” Roark said. “You get the call up, and yeah, you’re excited. But you don’t want to come up here and change the mentality that you had down in the minors or anything like that. You want to keep coming after them and not giving into anybody, no matter who it is.”

The Nationals trail by 41 / 2 games in the wild-card race with 11 to play. Their first victories against the Braves at Nationals Park this season ensured Atlanta could not clinch the National League East title until it leaves town.

“We have to show these guys that we can beat them,” Manager Davey Johnson said. “We don’t want them thinking they can just roll over us. That was huge in that respect.”

The Braves will eventually win the division. They have still beaten the Nationals 12 times in 18 games this year. But the Nationals ensured the Braves will not celebrate the title — the one the Nats are still technically defending — in their home stadium.

“They’ve broken our hearts all year,” Span said. “Of all teams, I don’t want to see them jump around on us. We know they’re going to the playoffs. We don’t want to see them jump around and celebrate on us. Let them go after this series and do that.”

The Nationals kept their postseason hopes alive, but they know any stumble may squash them at any time. “We understand that one or two losses might send us home,” Span said. And late Tuesday afternoon, it seemed a back-breaker had come.

The Nationals took an early 3-0 lead and nearly knocked out Braves starter Mike Minor in the first. But after Span’s single in the second, the Nationals would not record another hit until the ninth.

Meanwhile, the Braves chipped away until Evan Gattis crushed a go-ahead, two-run home run off Clippard in the eighth inning. In the top of the ninth, Harper fielded Elliot Johnson’s single to left and, upon seeing Johnson’s wide turn, chucked an errant throw behind Johnson. “I don’t know what he’s thinking,” Davey Johnson said. Elliot Johnson would steal third and score when a slow grounder rolled through Desmond’s legs.

“It’s only fitting the way it turned out that we got their superman closer,” Davey Johnson said. “It was worth it all.”

Trailing 5-3, the Nats’ chances seemed grim. Kimbrel had allowed seven runs all season. He had never allowed more than two in one appearance. Over the past two seasons, the Nationals had managed eight hits in 59 at-bats against him.

From the start, though, Kimbrel showed rare vulnerability. His fastballs to LaRoche hummed at 96 and 97 mph, a tick below his usual heat. LaRoche worked a 3-2 count and took ball four.

Wilson Ramos, catching his 24th consecutive game, chopped a grounder up the middle. Second baseman Elliot Johnson tried to flip it with his glove to Simmons at second base rather than take an easy out at first with the glacial Ramos.

With no outs and two on, Anthony Rendon tried to bunt, but pulled the bat back at pitches he deemed balls. They were not. Kimbrel eventually walked him on a 3-2 pitch. With the bases loaded, Chad Tracy scored one run with a groundout down the first base line.

Up to the plate came Span. Braves Manager Fredi Gonzalez kept the Braves’ infield back, which meant a grounder up the middle would tie the game. “I was a little shocked,” Davey Johnson said.

Span thought the Braves may walk him to set up a game-ending double play. The first pitch Kimbrel threw, a 97-mph fastball, was a ball, but close to the strike zone. Span knew Kimbrel wasn’t trying to walk him. “I was like, ‘Let’s go,’ ” Span said.

Kimbrel fired a 98-mph heater. Span bounced a groundball to Simmons. The ball skittered through his legs.

“I just missed it,” Simmons said. “No excuses. After I knew I didn’t catch it, it’s a blur.”

Third base coach Trent Jewett windmilled home Rendon. The Nationals’ dugout emptied, players streaming onto the diamond.

Scott Hairston reached Span first and jumped into his arms. Jayson Werth tore Span’s helmet off. Desmond yanked his jersey. Gio Gonzalez high-fived Span and yelled, “That’s what I’m talking about!”

For their 20th victory in 26 games, the Nationals had won in a manner that makes anything seem possible. Then they went out and won their 21st in 27.

“We just got rolling,” Haren said. “We knew we had a run in us. Unfortunately, it took five months, really, for us to get going. Hopefully it’s not too little, too late. But regardless, I’m really proud of the guys, the way we’ve come together.”