Cambridge completed their perfect sweep of cup victories over Northamptonshire sides to book a third round FA Cup tie at home to Millwall.
John Taylor's side had already beaten the NorthamptonÂ’s neighbours Rushden & Diamonds in both the Worthington Cup and the LDV Vans Trophy, and this backs-to-the-wall success over the Cobblers followed an LDV win here last month.
Substitute Omer Riza was the match winner, just as he proved in that earlier victory. This time, Riza came off the bench at half time to bundle in the goal 11 minutes from time, after a Tom Youngs cross had been nodded back across goal by David Kitson.
A delighted Taylor said: "This was almost a complete reversal of the 2-2 draw at our place the other week when we battered them in the first half and then they came back strongly." Taylor added: "I had to do something to make a change at half time. I felt sorry for Daniel Chillingworth as we sacrificed him to bring Omer on but I had to do something." Cambridge's other hero was goalkeeper Shaun Marshall who made crucial first-half stops. In the 17th minute Jamie Forrester was sent racing clear onto a Paul Harsley pass, but Marshall beat away his fierce drive.
And 15 minutes later Marshall again performed heroics to keep a Harsley strike out of the top corner.
As Northampton stepped up the pressure Marco Gabbiadini saw his shot on the turn stopped on the line by Luke Guttridge, while Andy Duncan headed against his own cross bar.
The second half followed the same if less intense pattern with Forrester and Harsley both denied by Marshall before Riza came up trumps with 11 minutes left.