Milwaukee added a pair of runs in the second, which the Giants offset by scoring twice in their half of the inning. San Francisco forged a 6-6 tie in the fifth on Andrew Susac's three-run homer.
"My bat speed is back up," said Susac, whose 2015 season ended prematurely with thumb and wrist injuries. "It's very confidence-boosting for me."
The Brewers' Michael Reed broke the tie in the eighth with a two-run double off Josh Osich. Ty Ross cut the Giants' deficit to 8-7 with an RBI single off Blaine Boyer in the bottom of the eighth.
In his debut with the Giants, Denard Span came off the bench and went 0-for-2 as the designated hitter.
Brewers starter Zach Davies left the game after working a scoreless inning. The right-hander said that he tried to establish his sinking fastball.
"I think I threw maybe threw one change-up," Davies said. "and tried to make sure I could move my sinker to both sides of the plate."
Up next for Brewers:Wily Peralta will take the mound when the Brewers host Taijuan Walker and the Mariners at Maryvale Baseball Park on Friday. Milwaukee manager Craig Counsell has yet to announce his pitching plans for the start of the season, but the spring rotation hints at Peralta leading the way, followed by Jimmy Nelson, Matt Garza, Taylor Jungmann and Chase Anderson in that order. Peralta was the Brewers' most valuable pitcher in 2014, when he won 17 games, but was limited to 20 starts last season because of an oblique strain. The 2:05 p.m. CT game will be webcast via brewers.com.