Bryce Harper watches his 100th career home run, a third-inning grand slam that powered the Nationals to a 6-2 win over the Braves. (Katherine Frey/The Washington Post)

History left a two-foot dent in the scoreboard at Nationals Park on Thursday afternoon. Just below Section 240, the board was flashing an ad for Good Humor ice cream at the moment of impact. The “r” disappeared, damage done by Bryce Harper on the reigning National League MVP’s 100th home run, which also happened to be the first grand slam of his career. The blast was the decisive blow in the Washington Nationals’ 6-2 win over the lowly Atlanta Braves.

“Perfect,” Harper said.

Perhaps caught up in the emotion, Harper thought he touched first base with his back heel on his home run trot but wasn’t sure so he turned back. The crowd erupted, and his teammates bounced in the dugout. Fans held up “100” signs and chanted “MVP!” until Harper reemerged for a curtain call, stood on the dugout’s top step and pumped his right fist three times.

“It was pretty awesome,” Harper said. “It was my mom’s birthday today, so being able to hit her a homer was definitely huge. I love these fans. I love this organization. It’s a thrill to be able to do it in front of the home crowd, and I’m glad I was able to do it here.”

The Nationals completed a four-game sweep of the Braves that gave them a 7-1 start. After his start was pushed back a day because of an illness, Stephen Strasburg tossed 7 2/3 strong innings, yielding just two runs while striking out seven. Wilson Ramos crushed a long home run himself and added an RBI double, continuing his hot start.

But Thursday’s game was yet another stamped by Harper. He came to the plate in the third inning against Braves starter Julio Teheran with two outs, the bases full and his team trailing 1-0.

Teheran’s 1-0 pitch came in at 91 mph, just below the belt and over the plate. It left the stadium considerably faster, making Harper the first player in MLB history to hit his first grand slam for his 100th home run. He joined Ryan Zimmerman and Ian Desmond as the only players to hit 100 home runs in a Nationals uniform, and he did so in fewer games.

“He’s young, and he’s got a 100 homers already,” Ramos said. “He’s helping the team a lot.”

At 23 years 181 days, Harper became the eighth youngest in major league history to eclipse the 100-homer mark. The players to do so at a younger age? Mel Ott, Tony Conigliaro, Eddie Mathews, Alex Rodriguez, Andruw Jones, Miguel Cabrera and Johnny Bench — three of whom are Hall of Famers and two more of whom could end up there. Harper hit the mark in his 518th game, which is 10 games faster than it took Babe Ruth.

“If you’re worried about 100, then you’re not going to get higher than that,” Harper said. “Not to take it for granted, I’m definitely excited, but I’m looking for many more to come. Being able to play behind [Strasburg] today, he did a great job. I’m just excited to contribute and get that [win].”

While Harper and Ramos provided most of the Nationals’ offense, Strasburg did his part on the mound. He had been sick all week and improved by Thursday, but Manager Dusty Baker admitted the team was unsure even midday whether he could start in the afternoon. Long reliever Yusmeiro Petit was the backup starter.

Baker told Strasburg before the game that, for an inexplicable reason, he has seen many players perform their best when sick. So when Strasburg walked the game’s first batter on five pitches but still escaped the inning unscathed, Baker was concerned.

“He came off like he was close to needing mouth-to-mouth,” Baker said. “I wasn’t going to give it to him. I mean, he was struggling big time, so we knew he was sick. . . . He was on guts. Big time. That was a gutsy performance.”

Despite medicine and an IV bag of fluids the day before, Strasburg displayed a powerful fastball, good tempo and wicked off-speed pitches, including an 88-mph slider he refuses to acknowledge exists. By pitching, Strasburg felt he sweated out the last bit of the illness and found a groove.

“I was going to go out there and give it everything I had, and that’s what I told them,” he said. “So whatever that was going to be, they were going to get it. I just kept telling myself just one pitch at a time. Kept reminding myself I was just a click away and it was all going to sync up eventually.”

Baker and the Nationals leaned on Strasburg, and the right-hander pitched into the eighth. When Strasburg left to a standing ovation, the Nationals had a commanding 6-1 lead.

The Nationals’ fast start has come primarily at the expense of the Braves (0-9), the only team in the majors without a win besides the Twins. Harper’s milestone home run was the big, final gut punch of the series for Atlanta. After the game, Harper said he took photos and traded a bat with the little girl in right field who caught his home run ball. The keepsake sat in a plastic box in his locker after the game.

“Look at every sport,” Baker said. “Guys that are considered the great ones — Michael Jordan, [Wayne] Gretzky, Jim Brown, Joe Montana — they have a flair for the dramatic. And I told him when he hit that home run: ‘Man, you’ve got a flair for the dramatic. You hit a grand slam on your 100th career home run. How many people have done that?’ ”