The loss goes to closer Salomon Torres, who fell to 5-3.
The game was tied 4-4 in the 7th and CC Sabathia appeared to have pitched out of bases loaded, 1-out jam. Lee grounded to shortstop JJ Hardy who flipped to second baseman Rickie Weeks for one, but Week's throw to first went wide left and out of play, allowing two runs to score.
The Brewers clubbed three home runs in the loss, all solo shots off the bats of JJ Hardy, Ryan Braun and pinch hitter Russell Branyan.
Ben Sheets pitches game two of the series tonight for the Brewers. The Cubs will counter with ace Carlos Zambrano.