Substitute Conor Chaplin sunk Northampton with a late winner as Portsmouth came from behind to beat the Sky Bet League Two leaders 2-1.
Portsmouth created a couple of early chances, both falling to Gareth Evans.
He just failed to get on the end of a dangerous cross from Caolan Lavery before squandering Marc McNulty's pass.
But it was the leaders who broke the deadlock in the 17th minute.
Marc Richards robbed Adam Webster only to be denied by keeper Brian Murphy but from Joel Byrom's ensuing corner Ryan Cresswell rose to powerfully head home.
Richards went close to rounding off a precise training ground routine as Northampton looked to double their advantage.
But Portsmouth equalised in the 32nd minute when McNulty broke away and crossed for Evans to head home at the far post.
The visitors survived a goalmouth scramble when Richards had a close-range effort blocked on the line after Byrom's corner caused problems.
But it took an excellent save from Adam Smith to thwart Lavery who was also denied by the Cobblers keeper moments later.
McNulty saw his header saved by Smith as Portsmouth ended the first half on top.
Smith was soon called into action after the restart to thwart the lively Lavery after he beat the offside trap, before saving a long-range effort from Evans.
Danny Hollands saw his drive deflected wide, Smith denied McNulty again and Evans fired wide as Portsmouth looked the more likely to score in a much tighter second half.
Northampton twice went close from Byrom corners, John-Joe O'Toole heading over and Cresswell glancing a header against the bar.
But Portsmouth won it in the 86th minute when a quick break ended with substitute Ben Tollitt seeing his shot parried by Smith and Chaplin scored from the rebound with his first touch.
Northampton substitute Dominic Calvert-Lewin then headed against the bar but Portsmouth held on to take all three points
Source : PA