The Rockies took the lead in the 13th inning and again in the 14th, but could not hold onto the advantage either time. The teams combined for 30 hits and five home runs.
Seth Smith tripled home a run for Colorado in the 13th to snap a 3-3 tie, but Yuniesky Betancourt hit a solo homer off closer Huston Street in the bottom half of the inning to keep the game going.
Dexter Fowler put the Rockies in front in the 14th with an RBI single, his first hit in seven at-bats, but Felipe Paulino (0-4) gave up the lead again.
Paulino retired Corey Hart to open the 14th, but Ryan Braun walked on a 3-1 offering and Fielder hit the first pitch he saw over the wall in right to end the contest.
Jason Hammel and Jason Giambi both homered off Milwaukee starter Matt Greinke. Casey McGehee hit a solo homer for the Brewers in the eighth to bring about extra innings.
The win went to Michael McClendon (1-0), the last of six Milwaukee pitchers, who gave up Colorado's go-ahead run in the 14th.
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