Harrison Dunk netted what proved to be the winning goal after just 54 seconds as Cambridge United gained a crucial victory in their battle against relegation from League Two.
5,520 watched the Cobblers try and recover from their slow start but Chris Wilder's side, disrupted by Chris Hackett leaving the field after half-an-hour, struggled to create clear-cut chances.
Results elsewhere were generally favourable to Northampton and they are five points off a play-off place with as many games to go.
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