Domonic Brown hit his NL-leading 19th home run, and had a two-run double in the eighth to account for all of the Phillies' runs.
Tyler Thornburg (1-0) pitched two innings of scoreless relief to earn his first major-league victory. A top prospect for the Brewers coming into the season, he joined the team Wednesday despite going 0-7 with a 6.75 ERA in 12 starts for Triple-A Nashville.
Francisco Rodriguez pitched the ninth for his fourth save in four chances.
With the score tied 1-1, the Brewers strung together four consecutive hits in the sixth off Kyle Kendrick (6-4) to score two runs and take the lead for good. Ryan Braun and Aramis Ramirez each singled, and Lucroy doubled to the base of the left-field wall, scoring Braun and giving the Brewers a 2-1 lead.
Carlos Gomez then singled to left, driving in Ramirez to make it 3-1.
Lucroy came into the game batting .253 after hitting .320 a year ago, but has been good in clutch situations. He was in a 3-for-18 slump before hitting the go-ahead double, but is batting .327 with 25 RBIs with runners in scoring position.
Kendrick allowed three runs on eight hits and a walk while striking out six in six innings.
The Brewers made it 4-1 in the fourth when Braun's single scored Segura from third off Michael Stutes.
The Phillies scored twice in the eighth off Brandon Kintzler. Michael Young, who had three singles, and Kevin Frandsen each singled.
Kintzler then struck out both Jimmy Rollins and Delmon Young, but Brown hit a floating double down the left-field line to score both runners and pull the Phillies within 4-3. Pinch-hitter Ryan Howard then hit a one-hopper back to Kintzler to end the threat.
The Phillies also threatened in the top of the sixth off Thornburg, but didn't score despite opening the inning with runners at second and third. The right-hander allowed two hits while striking out two in two innings.
Milwaukee starter Tom Gorzelanny, making a spot start, allowed only one run on five hits while striking out three in four innings.
Brown hit a 1-1 pitch into the Phillies bullpen in right field to make it 1-0 in the second.
Segura hit a 1-1 pitch into the Brewers bullpen in left field to tie it 1-1 in the third.
Braun had three singles and is 6 for 11 in the series.