Burton had two players sent off in the second half as their relegation fears increased with a comprehensive 3-0 defeat at play-off chasing Oxford.
Darren Moore was red-carded 11 minutes after the break for bringing down James Constable when the striker charged towards goal, and Tom Parks followed him to the dressing room after 77 minutes for a foul on Constable, his second yellow card of the game.
Steve MacLean scored twice for the Us, who moved to within three points of the play-off places.
His first goal took a wicked deflection to leave keeper Adam Legzdins wrong-footed, and that seemed to sum up the Brewers' luck.
Us keeper Ryan Clarke saved well from Parks and Calvin Zola at a corner as the visitors pressed strongly at the end of the first half, but Moore's dismissal early in the second effectively ended the contest.
Tom Craddock converted the resulting penalty and Oxford wrapped up the win in the final minute when MacLean tapped home from two yards.
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