Oxford United saw their lead at the top of the Blue Square Premier table cut to a point as they were held to a draw by battling Cambridge.
The hosts took an early lead when defender Brian Saah swept home Simon Russell's free-kick from six yards.
Oxford equalised courtesy of a neat turn and shot by Matthew Green.
But they were unable to find a winner and their night ended in frustration as Anthony Tonkin was sent off for pushing over Danny Crow as he ran in on goal.
With title challengers Stevenage Borough hammering Eastbourne 6-0, the league leaders will view this as two points dropped against a Cambridge side that has not won in 10 league games.
But the hosts proved a tough nut to crack as Rhys Day discovered early on when his effort at goal was blocked by Saah.
Adam Chapman was similarly frustrated when Paul Carden managed to get in the way of his shot from 12-yards.
Cambridge's defence continued to stand firm as Oxford's efforts to find the all-important second goal were repeatedly frustrated.
John Grant saw another shot blocked, Chris Hargreaves blazed over from inside the area and Sam Deering had an effort saved at point-blank range by Simon Brown.
But the home side also had their chances, as Crow saw a 25-yard shot and a free-kick spectacularly saved by Ryan Clarke, who also pulled off a fine stop from Josh Coulson's downward header.
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