MILWAUKEE -- After getting through one of his most challenging first innings this season, D-backs ace Zac Gallen’s confidence remained steadfast.

“He walked by me right there, and he said, ‘I got seven [innings] today,’” D-backs manager Torey Lovullo said. “‘Just give me 110 pitches. I’ve got seven innings for you.’”

That figured to be a best-case scenario for Arizona entering Wednesday, as Gallen looked to close out the Brewers in the National League Wild Card Series. But considering what transpired in the first inning, it was fair to wonder how long he would last.

Despite a rocky opening frame, Gallen got the D-backs exactly where they needed. He threw six strong innings, holding the Brewers to two runs as Arizona clinched a spot in the NL Division Series against the Dodgers with a 5-2 win.

“So, I lied, in that sense,” Gallen joked of falling an inning short of his promise. “But I just tried to settle in.”

Gallen allowed two runs on three hits and a walk in the first. The 32 pitches were his most in any first inning this season. His previous high was 28 against the Orioles on Sept. 3.

In fact, he only had three other innings with 30-plus pitches this year -- the fourth inning on May 19 against the Pirates (37), the fifth on Sept. 19 against the Giants (36) and the second inning on July 15 against the Blue Jays (31).

“He just probably didn’t sequence things right [in the first],” Lovullo said. “Made a couple of mistakes and they took advantage of them. It’s playoff baseball. Teams are ready to go.”

Before a reporter could find an adjective in a question about Gallen’s first inning, the right-hander obliged.

“Debacle,” Gallen said.

Christian Yelich hit a leadoff single and stole second base. After striking out William Contreras, Gallen fell behind Carlos Santana 3-0. He battled back to a full count but ultimately walked the Brewers’ first baseman in a nine-pitch sequence.

Mark Canha followed with a single, Sal Frelick hit a sac fly and Willy Adames hit an RBI single. Only after a check-swing groundout by Josh Donaldson did Gallen escape.

“I was pretty confident that if he got out of that first inning, that he would settle down,” D-backs third baseman Evan Longoria said. “I've seen him do it a couple of times throughout the course of the year -- obviously, not on this big of a stage. The first inning of a playoff game is a grind, man. The crowd is loud, they get a guy on and it's tough to settle yourself.”

Gallen was up against stiff competition. Freddy Peralta, whose 2.81 ERA in 13 second-half starts ranked seventh among qualified starters, was on the mound for Milwaukee. And with a raucous crowd behind him, he held the D-backs hitless for 4 2/3 innings.

Gallen had to settle in to give his team a chance.

“When you're playing on the road, you’ve got to try to stop momentum the best way you can,” Gallen said. “I just tried to give us a chance to win and keep us in the game, let the offense do what it’s doing.”

Gallen kept Milwaukee off the board for the next five innings, lending himself a hand defensively in the third. With two on and one out, Frelick hit a Statcast-projected 102.3 mph line-drive comebacker that knocked off Gallen’s glove. He retrieved it and started a 1-6-3 double play.

“I talked with [assistant GM Mike Fitzgerald],” Gallen said. “We're playing with house money. Nobody expects us to be here. Just try to make a play, keep us in the game. Thankfully, I guess it fell out of my glove, and we could turn two.

“I think that kind of settled the momentum on their side. We could take a deep breath, and then the offense did what they had to do.”

Alek Thomas broke up Peralta’s no-hit bid with a solo homer in the fifth. After Gallen set Milwaukee down 1-2-3 in the bottom half of the inning, the D-backs scored four times in the sixth.

Gallen struck out Adames on his 100th pitch in the bottom of the sixth to put a bow on his outing. And with the D-backs sweeping the Brewers, Gallen will be available for Game 2 in Los Angeles on normal rest.

“Just big pitches all the way around,” Longoria said. “I'm really happy that we were able to reward him offensively today. Him and Merrill [Kelly] did the same thing in that Houston series at the end of the year, and offensively we just didn't do anything to help them.

“Well-deserved win for him. Guy has been the anchor for us all year. Just super special, man.”