Pompey were left feeling extremely hard done by after defeat was snatched from the jaws of victory at Oxford.
Two late goals from the hosts saw the Blues beaten 3-2, having played much of the game with just 10 men.
Michael Jacobs cancelled out Luke McNally's opener before Joe Morrell was harshly dismissed for Danny Cowley's side following a challenge on Cameron Brannagan.
Ronan Curtis then fired the visitors into the lead after the break, but they could not quite hang on, as long-range strikes from Brannagan and Nathan Holland saw United claim the points.
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