Stephen Strasburg delivers one of his 99 pitches during a win at Nats Park. (Maddie Meyer/The Washington Post)

Stephen Strasburg emerged from the dugout in the ninth inning and jogged to the mound. As he fired his warmup pitches, the crowd cheered and stood. His first-inning entrance music, the White Stripes’ “Seven Nation Army,” blared over the speakers. In his young but well-documented major league career, the right-handed starter had never experienced this feeling.

He has struck out 14 batters in a game. He has also hit 100 mph on the radar gun. He was the first overall pick in 2009 draft. But until Sunday night, he had not stood on the mound in the ninth inning in the majors nor had he thrown a complete game.

In one of the most dominant and efficient outings of his career, Strasburg finally achieved a milestone he had so longed for as a member of the Washington Nationals. He entered this season without the chains of an innings limit and a growing leash from Manager Davey Johnson and vowed to be a workhorse in the starting rotation.

In the 68th start of his big league career, he tossed a brilliant shutout in a 6-0 victory against the Philadelphia Phillies at Nationals Park.

With the win, the Nationals swept the Phillies. Strasburg needed only 99 pitches to notch 27 outs — 10 of them by the strikeout — while pitching through groin discomfort in the second inning.

Strasburg dismantled the Phillies’ lineup with dazzling stuff. He fanned double-digit batters for the 10th time in his career. He gave up four hits and one walk.

As he talked with a MASN reporter following the game, teammates Tyler Clippard and Craig Stammen dumped a well-deserved tub of Gatorade on him.

“It’s something you try and do every time out, and you have to try and learn to go out there and get outs with less pitches,” Strasburg said. “That’s something I tried to set out doing at the start of the season here, and I knew it was only going to be a matter of time where I was gonna have one of those games where they hit it right to where we were playing and the defense made great plays.”

Strasburg even pitched around two minor nicks. While facing Domonic Brown in the second inning, Strasburg unleashed a 2-0 fastball far inside that aggravated his groin, which he said he tweaked earlier in the week.

Head trainer Lee Kuntz and pitching coach Steve McCatty emerged from the dugout and walked to the mound with Johnson close behind. Strasburg told Johnson his groin didn’t affect him when he flew open during his delivery. He remained in the game. And after walking Brown, he pitched as if nothing at all had bothered him.

The groin tweak “just helped me kind of take a step back and just go nice and easy and stay on line as long as I can,” Strasburg said.

In that inning, he fanned Darin Ruf on a perfectly placed outside fastball. He struck out Cody Asche on a blazing 97-mph fastball. He also hit 97 against Erik Kratz. He even had the awareness to work on a personal weakness. He stepped off the mound to hold Brown honest at first base. On the next pitch, catcher Wilson Ramos threw out Brown trying to steal second.

“He’s starting to figure out what kind of pitcher he is, how to work things, what to do,” third baseman Ryan Zimmerman said. “It’s fun to watch.”

After the walk to Brown, Strasburg struck out six of the next nine batters. He controlled his fastball while mixing his deadly curves and change-ups. He induced five groundouts, most in the later innings. Strasburg said he didn’t feel sharp while warming up before the game.

“When that happened in the past, I would just try and jam it down their throat and throw it as hard as I can,” he said. “I kinda learned that I didn’t really get much accomplished. So I think it just helped me take a step back and really focus on being nice and easy and hitting my spots.”

In the fourth, Strasburg squared around to bunt with two runners on and one out. Kyle Kendrick’s pitch hit Strasburg on his right forearm. Strasburg was headed to first when first base umpire James Hoye signaled that Strasburg had offered at the pitch and had to continue batting. Strasburg said the pitch only grazed him, and he wound up striking out.

No one in the bullpen ever stirred throughout the game, not even after the minor scares. Clippard said he noticed bullpen coach Jim Lett wander toward the bullpen phone. “We told him to sit back down,” Clippard said.

“He was very aggressive,” Ramos added. “That was the key for him today. Early in the game he was feeling something with his leg, so I was thinking he would be out of the game soon. But he is a horse. He fight today in all nine innings.”

The Nationals’ offense supported Strasburg in a way it had done so rarely this season. It staked him to a 1-0 lead in the first inning on an RBI single from Jayson Werth.

Another hot-hitting National was at the heart of the fourth-inning scoring. With Ian Desmond at second following a single and stolen base and Adam LaRoche at first after a walk, Ramos stroked a single to left to score Desmond. After two were out, Denard Span beat out a chopper over the mound. It was enough to plate another run and give Strasburg a 3-0 lead.

The Nationals loaded the bases again in the fifth. Werth and Desmond singled and LaRoche walked. Ramos then hit a groundball to second baseman Chase Utley, but catcher Kratz couldn’t handle Utley’s low throw, and Werth slid into home plate safely. With the awareness to see Kratz couldn’t find the ball, Desmond raced home and also slid safely under the tag. Werth walked off the field slowly, having bruised his knee, and was replaced in the seventh inning.

Sunday’s toppling of the Phillies was Washington’s first sweep since July 7, when it took three games from the San Diego Padres, and only its fourth sweep this season. It was also the first time the Nationals have swept the Phillies in Washington. And they did by riding the right arm of their ace, who notched his first win since that Padres series.

When Strasburg took the mound in the ninth, those of the announced 32,355 that remained in the unusual 5 p.m. game stood on their feet and applauded. He entered the frame with 90 pitches. He got Carlos Ruiz to fly out and Jimmy Rollins to ground out with a combined six pitches. All that remained was Kevin Frandsen.

“He was pounding the zone,” Johnson said. “When he starts sniffing that goal line, he’s amped up. . . . I haven’t seen him throw that hard all year consistently.”

Down 2-0 in the count, Strasburg fired a 93-mph fastball that Frandsen lined at Zimmerman, who made a slick back-handed diving stab. Strasburg’s game was complete. He tipped his cap toward Zimmerman. He high-fived Ramos and hugged. And then, near the dugout steps, got the Ga­tor­ade shower from his teammates.

“When a guy has a performance like that, you gotta get him,” Clippard said. “He kinda saw [us] in the dugout and he was like, No.’ And we were like, ‘Yes.’ So that was fun.”

“I was okay with the Ga­tor­ade,” Strasburg added. “I was just glad it wasn’t shaving cream.”