Northampton remain in relegation trouble after Danny East's strike saw Portsmouth win 1-0 at Sixfields.
East gave Portsmouth all three points in the fourth minute when he volleyed over goalkeeper Matt Duke from 10 yards after Jake Jervis helped on a free-kick.
Moments later Sonny Bradley headed wide from Danny Alfei's corner as Portsmouth took control of the early exchanges.
Northampton would have been on level terms had Emile Sinclair not completely missed a close range header from Evan Horwood's cross. Ricky Ravenhill fired over at the end of a good move and John Marquis was denied by Trevor Carson from a tight angle as Cobblers finally got into the game.
Darren Carter twice went close to an equaliser at the end of the first half, Carson saving his initial effort and then seeing the rebound blocked by Alfei.
Substitute Ivan Toney had a header saved by Carson after the restart and then shot over. In between Horwood fired over from a free-kick and then had a header saved by Carson as the Cobblers dominated the second half.
Portsmouth were content to defend their slender lead and offered little in attack in the second half but always looked dangerous on the beak, substitute Ricky Holmes shot over and Duke saved from Alfei.
Marquies was denied by Carson and substitute Kelvin Langmead had a shot blocked as Cobblers kept pressing for the equaliser but it never came.