Ryan Zimmerman hits an RBI single for the Nationals in an 8-3 victory over the Brewers. (Brad Mills/Usa Today Sports)

Milwaukee Brewers starter Matt Garza was so baffled and frustrated, he walked slowly off the mound toward the dugout and barely acknowledged his manager. He held his glove out and waved it at his side. Only one out into the bottom of the first inning Saturday, the Washington Nationals had a commanding lead and had chased the opposing team’s starting pitcher from the game.

Behind an offense that took advantage of a wavering starter, a little luck and a strong pitching performance from Tanner Roark, the Nationals clobbered the Brewers, 8-3, and moved back into a virtual tie with the Atlanta Braves atop the National League East.

The Nationals’ offense pounced on the Brewers for an early 5-0 lead and tacked on a few more runs. Roark plowed through one of the league’s best lineups with ease, producing a bunch of groundouts and pitching around a handful of base runners. Four outs into the game, the outcome already felt determined.

“Our goal as an offense is to try to get the starting pitcher out as soon as possible,” center fielder Denard Span said. “You never think the first inning. But seeing him walk off the field was definitely a good feeling because he still is a good pitcher.”

The entire Nationals lineup contributed to Saturday’s effort. Even the lone Nationals starting position player without a hit — Adam LaRoche, who is in an 0-for-17 slump — drew two key walks and delivered a run with a sacrifice fly in the sixth inning. Four Nationals — Ryan Zimmerman, Jayson Werth, Wilson Ramos and Span — each smacked at least two hits. Zimmerman and Ramos combined to drive in six of the team’s eight runs.

The Post Sports Live crew assesses the Nationals at the midway point of the season, and look at which players need to lead the team down the stretch to a potential National League East crown. (Post Sports Live/The Washington Post)

“Any time you can get [early] runs like that it’s nice,” Zimmerman said. “You can’t stop. The hardest thing to do and the biggest thing you need to do is keep piling on runs, especially with a team like that that can score runs. I thought we did a good job of doing that.”

But the best individual effort was Roark’s. Against the second-highest scoring team in the NL, he hardly budged. In his first start of the second half, Roark was in top form. He relied heavily on his sinker and on an 84-mph slider that he usually only sprinkles in.

Roark turned to the slider more than normal because the Nationals’ scouting report noted that the Brewers had success hitting fastballs and curveballs, two of Roark’s better pitches. And over the past handful of starts, Roark built new confidence in his slider after shifting his fingers on the ball to grip more of the laces, similar to fellow starter Jordan Zimmermann’s grip.

“It’s gone up velocity-wise and has been much sharper,” Roark said. “I was able to throw it a lot harder.”

For his third straight start, Roark delivered seven innings. He induced nine groundouts and allowed only one run on six hits. He walked only one batter and struck out five. He was in control from his first pitch to his last. He had no more than two Brewers on base at a time.

“He’s a really good pitcher,” Ramos said. “He’s got really good stuff. He likes to throw any pitch in any situation. That’s good for him. I like that a pitcher can throw any pitch in any situation.”

The Brewers scored their only run off Roark in the second inning. Jonathan Lucroy doubled off the right field wall with one out. He then stole third base and scored when catcher Ramos skipped a low throw past Zimmerman and into left field for an error. Beyond that, the Brewers had little chance against Roark.

With his pitch count at 100, Manager Matt Williams let Roark take the mound for the seventh inning. No one stirred in the Nationals’ bullpen, and the inning was his. On 12 pitches, he retired all three batters he faced.

As he walked into the dugout, fans stood and applauded, and Williams held out his hand for a handshake. In spring training, Roark fought for the final spot in the starting rotation. In late July, Roark’s 2.91 ERA is second-best in the rotation, just behind Doug Fister’s team-leading 2.90.

“One of the coolest things about baseball is the stories that you see develop, and Tanner is one of the best stories,” Zimmerman said. “Just shows you that if you keep working hard and keep doing what you’re supposed to do like he does it’ll pay off. He’s been great.”

Roark received a large early cushion thanks to an opportunistic Nationals lineup. LaRoche loaded the bases with a walk against Garza after Span’s single on a high fastball and Werth’s bloop double to right field. Zimmerman then laced a fastball to center for a two-run single.

The Nationals loaded the bases again when Bryce Harper drew a walk. Ian Desmond then hit a dribbling groundball toward shortstop Jean Segura for an infield single, which produced another run thanks to his speed. Ramos fell behind but then laced a slider into center field for a two-run single, giving the Nationals a 5-0 lead, and Desmond took third base on Carlos Gomez’s bobble.

“It was a lot of weight off my shoulders whenever we get five in the first,” Roark said.

The mess of an inning only got worse for the Brewers when Manager Ron Roenicke emerged from the dugout to remove Garza from the game with Roark due up next. Garza was clearly furious about his removal after only one out. He made little eye contact with Roenicke and began his deliberate strut to the dugout.

A few of the Nationals’ hits off Garza were dribblers, and they fouled off 13 of his 42 pitches, so he suffered from some bad luck, but his command wasn’t sharp. The Brewers then replaced Garza with Marco Estrada, a starter stashed in the bullpen with a 4.95 ERA. Estrada notched the next two outs, the first inning finally over after 43 minutes.

“We got some lucky hits and we turned them into big hits to get more runs,” Zimmerman said.

After their biggest first inning of the season, the Nationals added three more runs. By the time Jerry Blevins took the mound in the eighth inning with an 8-1 lead, the liveliest thing in Nationals Park was fans doing the wave. Blevins gave up a two-run home run to Ryan Braun, who drew a loud chorus of boos, but the runs mattered little.

Long before they added more runs and Roark cruised, the Nationals took a big early lead and didn’t look back.