When the inning started, Fielder was far from the first name on the list of potential players to give Milwaukee the win. The Brewers started with the seventh spot in the lineup, Mike Rivera, who struck out to begin the inning.
But Craig Counsell hit a pinch-hit single and Mat Gamel drew a walk, before Corey Hart lined a single to center field to cut the deficit in half.
J.J. Hardy also singled, scoring Gamel, before Fielder ended one of the team's most exciting games this season.
"It's up there, especially coming back off a guy like Brian," Fielder said of calling Saturday's win one of the best of the year. "He's a great closer and to be able to come back like that is a good thing and it gives us a good feeling."
Fielder's game-winning knock gave Wilson (2-4) his fourth blown save of the year, and also kept Brewers closer Trevor Hoffman (1-1) from picking up his second loss of the season.
Hoffman entered the game in the top of the ninth with the game tied, 4-4, but allowed three straight singles to load the bases. Both Bengie Molina and Edgar Renteria hit sacrifice flies to give San Francisco the lead.
"It wasn't a very good night for closers on either side, to say the least," said Brewers manager Ken Macha, who also added he may have to reconsider putting Hoffman in the game during non-save situations.
The night was full of comebacks for the Brewers.
Seth McClung, making his first start of the season, allowed three runs, throwing 77 pitches in four innings. Two of the runs came in the second inning, when the Giants' first four batters knocked out hits.
"I really felt sharp, to tell you the truth, especially in the second inning," McClung said. "Those guys hit some really good pitches off of me."
Pablo Sandoval also hit two solo home runs before the Brewers made their first comeback in the sixth inning on back-to-back home runs by Fielder (three-run homer) and Casey McGehee.
Fielder's home run came off Giants starter Barry Zito, who held Milwaukee to just two hits prior to the homer.
It looked as if San Francisco would bounce right back in the bottom of the seventh, when Nate Schierholtz drove a ball to deep center field, but Brewers center fielder Mike Cameron robbed Schierholtz of a home run.
"I had a good beat on it," Cameron said. "It was on that little short side of the wall and I got my old legs to get up and go."
Macha called Cameron's catch "tremendous," and said between that catch, Fielder's walk-off hit and the excitement, the 42,065 fans in attendance got their money's worth Saturday.
"I hope the fans had a good time tonight, I really do," Macha said. "They saw a lot of stuff that was just something else. You come to the ballpark to get that kind of entertainment every night."
Saturday's win was the third in Milwaukee's last four games, coming after a stretch where the Brewers went 4-10.
Between some of the big ninth-inning hits by players who have been slumping -- Hart broke an 0-for-19 streak and Hardy (.224) has been mired in a season-long struggle -- and the never-say-die attitude, Cameron said he hopes the cards are in order for the Brewers to build off their two-game winning streak and go for the series sweep Sunday.
"That's a very big win for us," Cameron said. "Hopefully we can get on the good side of snowballing -- we've been on the bad side of the snowball a little bit. Hopefully we can get some good things going and roll it a little bit more to and see what happens tomorrow."