Eastbourne Borough suffered a massive blow on the stroke of half time as Joe Benjamin pulled up with a hamstring injury.
Benjamin, who only rejoined Borough on loan on Friday, collapsed as he chases a ball down the line from Matt Crabb and is unlikely to return after the break.
It is a huge setback for Borough but it could have been worse with only two goalline clearances just before the interval keeping them on level terms.
Neither side had threatened until the 44th minute when Cambridge suddenly turned up the pressure.
They looked certain to score when Robbie Willmott's cross picked out Danny Crow at the far post. He nodded the ball down to Sam Ives but a combination of Gary Elphick and Neil Jenkins managed to block his shot on the line.
The resulting corner found Rory McAuley whose goalbound volley was kicked away by Matt Crabb on the line.
Cambridge's only other real chance came when Marc Pullan carelessly gave the ball to Crow on the edge of the area but the United striker shot wide.
Borough have passed the ball nicely at times but offered little threat going forward with a Neil Jenkins shot from 25 yards which flew over the bar their best effort.
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