Oxford United are now left looking realistically at the play-offs after Stevenage beat them at Broadall Way tonight to move eight points clear of them at the top.
In a full-bloodied battle between the top two sides in the Conference, Borough won it with a penalty seven minutes into the second half.
Until then, United had given as good as they got against a Stevenage side unbeaten a home since November 2008.
But their response to the goal looked like that of a team drained, physically and emotionally, as they have seen their title hopes slip away.
Apart from a free-kick by Simon Clist, who had given away the penalty, which was tipped over, they seldom troubled Chris Day in the Stevenage goal.
Having led the table for more than half the season, it is a bitter pill for Oxford fans to swallow knowing it will take a miracle for them to now overhaul either Graham Westley's team, who were chalking up an eight successive win, and Luton, who moved above Oxford into second place in the table by seeing off Salisbury.
The pitch was very wet after steady afternoon rain, and one goalmouth, in front of the end where the Oxford fans were packed, was a real mess.
Chris Wilder made the brave decision to recall Mark Creighton and drop Rhys Day, and he also gave Matt Green a rare start following his late headed winner against Gateshead at the weekend.
Andy Drury tried to run at Oxford straight from the kick-off, and he was scythed down by Chris Hargreaves.
United had a let-off in the second minute when, from Ronnie Henry's right-wing cross, that Creighton couldn't clear, David Bridges screwed his shot wide of the right hand post from a great position.
Henry delivered another telling cross moments later, and Tim Sills met it with a flicked header that Ryan Clarke managed to save above his head.
Jake Wight produced a great piece of defending against Yemi Odubade, earning a goal kick when Odubade had looked like beating him with his pace.
And Creighton showed he meant business with a hard, but fair, challenge on Mitchell Cole, winning the ball but leaving the Stevenage player writhing on the ground. Typically, Graham Westley and his players surrounded the fourt official to protest about the tackle.
Cole was not able to continue for long, however, and Chris Beardsley replaced him on 20 minutes.
Oxford had set out to be strong and they looked to have unsettled the home side, who were not putting together any moves of note.
And when they won a free-kick in a dangerous positin 20 yards out, after Creighton was penalised for pushing down on a opponent when heading clear, United's wall did its job.
Constable was furious when the referee, who had been unable to keep up with play, didn't award a free-kick when Jon Ashton pushed down on the back of his neck to stop him charging forward.
Adam Chapman brought the keeper to his knees to save a low 35-yard free-kick, and Sam Deering tied a curling shot from 20. Both were comfortable for Chris Day.
At the other end, Creighton was getting his head on everything, and his teammates were making some terrific blocks.
When Drury dribbled Adam Chapman in the box, and then past the goalkeeper, it looked like he had to score, but he ran out of room.
Sills then headed against the top of the crossbar from Drury's cross at a well-worked free-kick.
The game opened up much more in the second half, which was only six minutes old when Sevenage won a penalty, and took the lead.
Clist fouled Beardsley with a tackle from behind inside the box and the ref pointed immediately to the spot.
Scott Laird then sent Clarke the wrong way from the penalty to leave United with an awful lot to do.
Beardsley broke free through the inside right channel, but Clarke kept out his shot with a smart low save.
United's keeper made another agile save from Beardsley and as it started to become end-to-end, with United sending on Alfie Potter and throwing more bodies into attack, Simon Clist forced Chris Day into a fingertip save from a free-kick.
The Oxford players' heads had dropped a bit after the goal, but their fans kept cheering them on.
And they were nearly rewarded with an equaliser as Matt Green rose well to meet Anthony Tonkin's cross from the left, but planted his downward header wide.
As the Stevenage players came off amid joyous scenes, United's trooped off heads down, dejected and forlorn.
Stevenage Boro: Day 6, Henry 7, Ashton 7, Roberts 8, Laird 7, Cole 6 (Beardsley 6, 20), Bridges 6, Drury 8 (Long 74), Bostwick 7, Sills 6 (Byrom 81), Odubade 6. Subs not used: Bayes, Griffin. Booked: Bostwick, Long.
Oxford Utd: Clarke 6, Chapman 7, Creighton 7 (Day 6, 69), Wright 8, Tonkin 6, Deering 6, Bulman 6, Hargreaves 7 (Potter 63), Clist 5, M Green 6, Constable 6. Subs not used: Turley, Chalmers, Grant. Booked: none.
Referee: S Creighton 7.
Att: 5,744 (approx 1,800 from Oxford).
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