Alfie Potter's late winner fired Oxford United to only a second victory in nine games against struggling Salisbury.
United dominated the game but had to wait until the 85th minute when Potter headed past James Bittner from Mark Creighton's flick-on.
The goal came moments after the substitute winger had an effort ruled out for offside following Bittner's parry from Adam Chapman's shot.
Matthew Tubbs had City's best chance but his speculative effort went over.
United topped the table last month but their disastrous run sees them trail Stevenage by eight points, and second-placed Luton by six.
Their play-off place will be guaranteed if they pick up one more point in their remaining five games - or if AFC Wimbledon fail to win one of their remaining games.
Salisbury are just four points clear of the relegation places after Potter's late intervention.
Oxford Utd: Clarke, Batt, Creighton, Wright, Sandwith (Deering 54), Francis Green (Potter 73), Chapman, Bulman, Tonkin, Constable, Midson. Subs Not Used: Turley, Day, Cook.
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