MILWAUKEE -- For all the times Madison Bumgarner has carried the San Francisco Giants on his back, his teammates certainly returned the favor Monday afternoon under the roof in chilly Milwaukee.

While Bumgarner battled through the last remnants of flu symptoms, the Giants' offense produced 14 hits including five home runs in a 12-3 opening day laugher over the Milwaukee Brewers at Miller Park.

"It's unbelievable," Bumgarner said. "It was really fun to watch."

San Francisco's ace left-hander took the mound Monday despite having spent the weekend battling a flu bug that left him with a fever and chills Saturday night. He still wasn't quite at full strength when the game got underway.

It was obvious, too, as Bumgarner allowed a leadoff single then a walk to Jonathan Villar before recording his first out on a Ryan Braun fly ball to center.

The situation worsened when he walked Jonathan Lucroy then Chris Carter to give Milwaukee a 1-0 lead, prompting a visit from Bochy. He escaped any further damage when Aaron Hill bounced into an inning-ending double play.

"I think without question it was affecting him," Bochy said. "But he found a way to get through it. He gave us five and that's about what I was hoping for when the game started. I figured he was a little washed out.

"First inning, that big double play, then (Angel) Pagan throwing a guy out at home. ... Those are big outs in a game like that. Both starters were struggling but he found a way."

Matt Duffy wiped out that early deficit in the second, driving in a pair with a single to left and after Scooter Gennett evened the score at 2-2 with a leadoff homer in the bottom of the inning, Denard Span broke the tie with an RBI single -- his first of five RBIs on the day.

The Giants never relinquished the lead after that.

Duffy hit a two-run homer in the fifth that made it a 7-3 game, giving Bumgarner all the cushion he'd need.

"Days like today, it helps a lot when you don't have your best stuff and you can still challenge guys and know it's going to take a lot to change the course of the game the way they were just putting up runs," said Bumgarner, who went six innings and held the Brewers to just three runs despite allowing five hits and five walks while striking out six.

Brewers starter Wily Peralta struggled in the first Opening Day start of his career. He retired the Giants in order in the first but would last just four innings and was charged with five runs -- four earned, thanks to an error on Aaron Hill -- on six hits and a pair of walks with six strikeouts.

"The first inning was really good stuff but they just put some good at-bats against him," Brewers manager Craig Counsell said. "What they are good at is putting the ball in play on tough pitches with two strikes.

"He made a couple of mistakes. He made an 0-2 mistake to Span. Posey kept an at-bat alive with just making contact with the ball. Then they just hurt him with men in scoring position," Counsell added.

Things didn't go much better for the Brewers' bullpen, especially late.

Carlos Torres served up Duffy's two-run home run but the Giants turned the game into a rout in the eighth when Span, Joe Panik and Buster Posey hit three consecutive home runs off rookie right-hander Ariel Pena.

"When you are in a game like that, you need your pitcher to keep getting outs," Counsell said. "It is important for the rest of the bullpen, for how the game looks tomorrow and how the game looks the next day. Frankly, I was hoping to get Pena through the rest of the game. I think he ran out of gas with that Span homer."

NOTES: Brewers RHP Matt Garza was "unlikely" to make his scheduled start Wednesday, manager Craig Counsell said before the game. Garza experienced shoulder stiffness in an exhibition start on Saturday against Double-A Biloxi. Counsell said Garza would be examined by the team's medical staff Monday. ... Giants LHP Madison Bumgarner made the Opening Day start despite battling flu symptoms. Bumgarner made four Cactus League starts for San Francisco, going 0-1 with a 9.56 ERA in 16 innings. ... The Giants' 12 runs marked their highest opening day output since plating 13 on April 5, 1983 against San Diego.