A return to League Two action did not equate to a return to form as Pompey were soundly beaten at AFC Wimbledon.
Trevor Carson got the travelling fans cheering when he saved a Harry Pell penalty midway through the first half.
But centre-back Andy Frampton netted a brace for the Dons after twice being left unmarked to score.
Sammy Moore then bagged an impressive third for the visitors before Michael Smith completed the scoring late on.
Pompey did not create too much at the other end, although they did twice hit the post in the second half.
Guy Whittingham made five changes from the side that exited the Johnstone’s Paint Trophy at Newport during the week.
David Connolly and Patrick Agyemang started up front, while there were returns for Jed Wallace, Simon Ferry and Johnny Ertl in midfield.
Neither side grabbed the game by the scruff of the net in the opening stages, although Ross Worner was forced into action when Danny East’s cross struck Agyemang and bounced goalwards.
There was certainly a lack of fluidity, which probably contributed to the surprisingly flat atmosphere at a packed-out Cherry Red Records Stadium.
Connolly and Wallace did link up neatly just outside the Dons box on 19 minutes, but the latter’s back-heel was cut out by a home defender.
But it was a couple of minutes later when the ground came to life after the hosts were awarded a penalty.
Carson tore of his line to try to beat Kaid Mohamed to Smith’s flick, but only succeeded in taking out the Wimbledon winger.
Referee Rob Lewis pointed straight to the spot and booked the perpetrator, but Pompey’s keeper got down quickly to his left to palm Pell’s spot-kick to safety.
Carson then made an even better stop at point-blank range to prevent Pell from converting the loose ball.
But Pompey were dealt a blow on 29 minutes when Agyemang was forced to limp off, with Ryan Bird replacing him up front.
And things soon got even worse as AFC Wimbledon broke the deadlock after Bondz N’Gala had sliced a clearance out for a corner.
That was only cleared as far as Callum Kennedy who swung a cross towards Mohamed in the box.
Carson got a hand to his effort, but that just left an unmarked Frampton with a simple close-range finish.
Alan Bennett also had the ball in the net just before the break after Carson had failed to hold a free-kick, but the linesman had spotted a foul on the keeper and the goal was ruled out.
Half Time: AFC Wimbledon 1 Pompey 0
The first half had – penalty incident and goal aside – not contained much to excite any neutral in the crowd.
And, unfortunately, the second period started in much the same fashion, with neither side creating any openings.
Whittingham decided to swap steel for creativity in his midfield on 56 minutes, with Ertl making way for Thery Racon.
But it was the hosts who were next to threaten and Sonny Bradley did well to turn Mohamed’s cross behind for a corner.
Pompey finally created their first opening on 63 minutes, but Andy Barcham’s shot curled narrowly wide after the winger had been teed-up by Connolly.
The same player almost grabbed an equaliser moments later, with his snatched effort rolling agonisingly against the post.
Bird’s afternoon was then brought to an early conclusion as he received a knock on his head while defending a corner and was replaced by Tom Craddock.
And the Dons added insult to injury by doubling their advantage on 73 minutes, with Frampton again left in too much space to convert a Kennedy corner.
Their third goal was a bit more special, however, with Moore’s cleanly struck half-volley from distance leaving Carson helpless.
A long Dons clearance was then flicked on for Smith to race onto and slam a shot home to complete a miserable afternoon for Pompey.
AFC Wimbledon (4-1-4-1): Worner; Fuller, Bennett (c), Frampton, Kennedy; S.Moore; Porter, Pell, Sweeney (L.Moore 74), Mohamed (Francomb 85); Smith
Goals: Frampton 33, 73, S.Moore 82, Smith 90
Booked: Pell, Fuller
Subs not used: Brown, Weston, Fenlon, Saint-Luce, Midson
Pompey (4-4-2): Carson; East, N’Gala, Bradley, Devera; Wallace, Ertl (c) (Racon 56), Ferry, Barcham; Connolly, Agyemang (Bird 29 (Craddock 70))
Booked: Carson, Ferry, Bradley
Subs not used: Smith, Moutaouakil, Painter, Cooper
Referee: Rob Lewis
Attendance: 4,605 (802 Pompey fans)