With the Padres trailing 3-1 in the sixth, pitcher Andrew Cashner and Everth Cabrera laid down consecutive bunt singles and one out later Chase Headley drew a walk from Brewers starter Tom Gorzelanny to load the bases.
Quentin followed by slicing an 0-1 pitch off the right-center field wall, chasing home Cashner and Cabrera with the tying runs.
After Jedd Gyorko fanned for the second out, Guzman scorched a pitch down the left-field line, scoring Headley and Quentin.
The rally handed a victory to Cashner (6-5), who allowed three runs on three hits over six innings. He walked three and struck out three.
Dale Thayer and Luke Gregerson each supplied a scoreless relief frame and Huston Street notched his 17th save as San Diego claimed the opener of a four-game series.
Gorzelanny (1-4) surrendered five runs on five hits in a 5 2/3-inning stint.
Jean Segura knocked in a pair of runs for Milwaukee, which lost all-star outfielder Ryan Braun for the season to a drug suspension before the game.