Pompey saw off a late spell of pressure to secure a valuable League One victory at Oxford.
The only goal of the game came early in the second half, with Harvey White marking his first Blues start by converting a Callum Johnson cross.
And the visitors looked to be seeing things out comfortably, only for a United rally in stoppage-time that saw Craig MacGillivray make three key saves.
Fans can watch the key moments (top video) or register with iFollow for extended highlights (middle video) and a full match replay (bottom video).
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