"I don't try to hit for power. I just try to hit the ball hard," Fielder said. "I am just trying to win the competition. He's a good pitcher. It wasn't about feeling good or feeling locked in."
Fielder, the defending National League home run champion with 50 a year ago, went homerless in his first 14 games this season and hit just one in his first 68 at-bats before Wednesday. Last season, he had just two homers in his first 69 at-bats over 18 games.
"I have got a job to do," Fielder said. "No matter how I feel, I have got to try to just win that battle every time."
Hamels (2-3) lost despite striking out a season-high 11. He gave up eight hits and five runs in seven innings.
Mitch Stetter (1-0) pitched a scoreless seventh and eighth to get the win as the Brewers overcame Chase Utley's major league-leading 10th homer.
Derrick Turnbow pitched the ninth for his first save in exactly a year.
For Turnbow, a 2006 All-Star who has been booed regularly since manager Ned Yost took the closer's job from him and gave it to Francisco Cordero and then Eric Gagne, it was a special moment.
"It's just amazing that Ned had the confidence in me to put me out there in that situation," he said. "It's unbelievable. It's been a long time, and I always hoped that one day I would get a chance to get back out there and have the opportunity and hopefully get one. I missed it a lot."
Yost said Gagne, who pitched four straight days through Monday, was unavailable. Turnbow said he didn't know he would be the closer Wednesday until after Fielder hit his home run.
"I have faith in him," Yost said. "And as long as I have faith in him, he doesn't have to worry about nothing."
Pat Burrell and Greg Dobbs also homered for Philadelphia.
Hamels gave up three runs in the first inning but cruised until the eighth, when Ryan Braun doubled to lead off the inning and Fielder followed with a line-drive homer on a 1-2 pitch to right.
"When the guy hit the double, I definitely was going to leave him in there on Fielder," Phillies manager Charlie Manuel said.
"Fielder had a good at-bat. He fouled off some pitches, and he got him. But at the same time, I liked (Hamels), I still liked him in the game. I liked him there, and I wanted him to stay there."
Hamels said he hung a changeup to Fielder.
"He's a great hitter," Hamels said. "I definitely wanted to avoid his strong zone, which is the middle in. You know just trying to keep him off balance is real tough."
Fielder also homered in the first inning for his ninth career multihomer game.
"He hit 50 home runs last year," said Brewers starter Dave Bush, who gave up eight hits, four runs and the three homers in six innings. "He's not going to hit five all of this year."
The Brewers hit Hamels early, with four of the first five batters collecting hits on Hamels' first 16 pitches.
Rickie Weeks doubled to start the game and scored on Gabe Kapler's single. Fielder's homer into the Phillies' bullpen in right field made it 3-0. Hamels also had a throwing error in the inning on an errant pickoff throw.
Utley made it 3-1 with his homer in the third, giving him homers in six of his last seven games. Dobbs' two-run, fifth-inning homer tied it at three, and Burrell made it 4-3 in the sixth with his eighth homer.
The game also marked the return of Geoff Jenkins to Milwaukee. Jenkins played 10 seasons for the Brewers before signing with the Phillies in the offseason.
Notes: Burrell tied the Phillies' franchise record for RBIs in April with 23. ... The Brewers activated CF Tony Gwynn from the 15-day disabled list Wednesday. Gwynn missed 12 games with a strained left hamstring after being placed on the DL on April 9, retroactive to April 4. ... CF Mike Cameron, suspended 25 games to start the season, is scheduled to join Triple-A Nashville on Thursday for a five-game rehab stint and make his Brewers debut next Tuesday.
