Pompey’s inconsistent start to the season continued as they were beaten 3-1 by Northampton at a sodden Sixfields Stadium.
The visitors were well below-par in the first half and trailed to a Chris Long strike and Matt Clarke’s unfortunate own-goal.
But they dominated after the break and Matty Kennedy deservedly pulled one back, with several more chances to level the game soon following.
A healthy travelling support were not able to celebrate a leveller, however, and Long netted again for the hosts to wrap up victory for the Cobblers.
Kenny Jackett made one enforced change from the side that won at AFC Wimbledon three days earlier.
Oliver Hawkins suffered a nasty head wound in that fixture and was replaced up front by Conor Chaplin.
A mixture of driving rain and strong winds made conditions difficult for both sets of players throughout the contest.
Pompey carved out a couple of half-chances in the early stages before Kennedy curled a 20-yard effort narrowly over the bar.
But it was the hosts who almost broke the deadlock on 15 minutes, with Matt Grimes’ free-kick causing confusion between Luke McGee and Clarke before eventually swinging out of play.
It was only a brief respite for the Blues, however, as the Cobblers did forge ahead just a few moments later.
Shaun McWilliams slipped a fine pass through to Long and although McGee came off his line to narrow the angle, the striker neatly slotted home.
Unfortunately, McWilliams’ evening was soon cut short, as he went down injured and had to be stretchered from the pitch.
That did not stop the Cobblers’ charge, though, and some poor defending allowed them to double their advantage on 32 minutes.
Grimes delivered a free-kick from wide on the right and no Pompey player was able to get it clear, with the helpless Clarke only able to turn the ball into his own net at the far post.
It seemed to stun the Blues into action and they had a few opportunities to reduce the deficit before the interval.
Northampton almost provided a helping hand when Ash Taylor diverted Jamal Lowe’s cross against his own post.
Stuart O’Keefe then tried a long-range effort deep into stoppage-time that Matt Ingram did well to tip over the bar.
Northampton Town 2
Pompey had been poor before the break, but whatever Jackett told them during the interval seemed to do the trick.
They came roaring out of the blocks and soon reduced the deficit when O’Keefe teed-up KENNEDY for a fine finish into the bottom corner.
Adam May then had an effort deflected over before Lowe arrived at the far post to head Damien McCrory’s cross just past the post.
The Blues kept pouring forward and Brett Pitman received the ball inside the box, only for some decent Cobblers defending to prevent him getting a shot away.
Lowe then had another excellent opportunity to draw the visitors level, as there was time for him to bring Kennedy’s cross under control before a wild shot that flew wide.
Jackett made his first switch of the evening just before the hour mark, as May made way for Ben Close in midfield.
Lowe seemed to be involved in everything that was happening and soon after being booked for a foul on Daniel Powell, his low centre was deflected into the arms of a grateful Ingram.
It was pretty much one-way traffic and when McCrory’s free-kick was only partially cleared, Kennedy sent the ball back into the box, where it was turned wide by Chaplin.
But rising hopes were dashed on 70 minutes when Northampton scored against the run of play to strengthen their advantage.
There was some good work on the right by Brendan Moloney and Crooks, with the latter’s cross flicked on by Alex Revell for Long to lash home his second of the night.
Pompey, to their credit, kept trying to get the ball into their opponents’ box, but the Cobblers were holding firm at the back.
They forged a decent opening late on, however, when McCrory whipped a fine free-kick into the box, where Christian Burgess’ header was kept out by Ingram.
And the keeper made an even better stop on 87 minutes, flinging himself to the right to palm clear O’Keefe’s cleanly-struck attempt and ensure there was no late drama.
Northampton (4-4-2): Ingram; Moloney, Taylor, Pierre, Buchanan (c); Crooks, McWilliams (Kasim 29), Grimes, Smith (Powell 56); Revell (Richards 78), Long
Goals: Long 18, 70, Clarke 32 (og)
Subs not used: Cornell, Barnett, Poole, Waters
Pompey (4-2-3-1): McGee; Thompson, Burgess, Clarke, McCrory; May (Close 57), O’Keefe; Lowe, Pitman (c), Kennedy; Chaplin
Goals: Kennedy 49
Booked: Thompson, Clarke, Lowe
Subs not used: Bass, Talbot, Donohue, Bennett, Naismith, Main
Referee: Darren Drysdale
Attendance: 5,855 (1,153 Pompey fans)