Third Division Cambridge looked like shocking Northampton for the second time in a month when they stormed into a 2-0 half-time lead in a thrilling cup tie.
But just as United hit back from behind in their 4-2 LDV Vans trophy victory at Sixfields, the Cobblers came back this time.
Young Cambridge defender Adam Tann got the U's off to a great start with a fifth minute header, adding a similar goal just before the interval.
Darryn Stamp pulled a goal back in the 55th minute and Chris Hargreaves forced the replay seven minutes from the end.
As Cobblers boss Kevan Broadhurst admitted they could not cope with the pace of Cambridge wingers Shane Tudor and Omer Riza in a first half dominated by the home side.
Once they rearranged and played a tighter game after the break it was their turn to take a grip on the game.
Cambridge will look back on more missed chances, a couple in the first half when Dave Kitson and Luke Guttridge went close and the crucial moment just before the final whistle.
Riza raced clear, but with goalkeeper Lee Harper at his mercy fired right at him.
Broadhurst commented: "I thought 'oh no' when their lad went through after all the hard work we had done to get back. It would have been rough justice on us to have lost it with the last kick." United boss John Taylor could not hide his disappointment: "We could have had four or five and wrapped it up by half time. They got a goal back when they needed it and we went into our shell a bit after that." TannÂ’s first header was set up by a Riza pass, their second by a well-placed cross from Paul Wanless.
Stamp headed home a Marco Gabbiadini cross in the 55th minute and Hargreaves racing through to hit a 15-yard shot in the 83rd minute.
Wanless had to go to hospital with facial injuries after being stretchered from the pitch following a clash of heads in stoppage time.