John Mousinho experienced his first disappointment as Pompey head coach with a 2-1 defeat at Peterborough.
Jonson Clarke-Harris bagged a brace in the first half – the second from the penalty spot – to leave the hosts cruising.
The Blues battled back after the break, though, and Reeco Hackett’s effort was controversially disallowed.
A late Nathan Thompson own-goal did ensure a tense finale, but the visitors were unable to find an equaliser.
Mousinho made two enforced changes from the side that won on the road at Fleetwood earlier in the week.
With Ronan Curtis injuring his ankle and Joe Morrell suspended, in came Dane Scarlett and Louis Thompson.
The latter was making his first start since suffering a broken leg in August and was up against brother Nathan, who was selected by the hosts.
Peterborough shaded the opening exchanges and yet it was the visitors who had the first decent chance to break the deadlock.
It came from a long Marlon Pack throw that was launched into the box, with the ball hitting Ryley Towler and flying inches past the post.
But instead it was the hosts who did forge ahead soon after, when Kwame Poku cut in from the right and hit a low pass into the box.
Pompey were unable to clear the danger and ball rolled through to Clarke-Harris, who was left with a simple close-range finish.
The visitors tried to find a response and Pack’s free-kick somehow evaded everyone on its way out of play, while a corner from the same player was headed over by Towler.
Zak Swanson then sent a teasing delivery right into the mixer, where a diving Colby Bishop could not quite make a connection.
And instead the Posh had the chance to take a commanding lead into the interval when Swanson was penalised for bringing down Ephron Mason-Clark just inside the area.
Clarke-Harris stepped up to take the spot-kick and slammed home an unstoppable effort past Matt Macey.
Half Time: Peterborough 2 Pompey 0
Pompey might have fallen further behind just after the restart after an unfortunate slip from Swanson.
That allowed Mason-Clark to cut the ball back for Joe Ward inside the box, but his fierce strike was beaten away by Macey.
And the keeper came to the rescue again moments later, this time using his legs to deny Ward following a quick Posh counter.
Still the hosts kept coming forward and a Mason-Clark cross from the left was nodded wide by Jack Taylor from just a few yards out.
Thompson received a booking for the Blues after fouling Mason-Clark just before the hour mark and that was his last contribution, immediately making way for Hackett.
Pompey had their first shot on target on 67 minutes, with Scarlett bringing the ball down smartly and hitting a low shot that Will Norris got down to save.
A passage of neat passing play then ended with former Posh midfielder Ryan Tunnicliffe having a shot deflected past the post.
It was the visitors’ best phase of the game and Mousinho added some fresh attacking legs by replacing Owen Dale with Michael Jacobs.
There was then a moment of controversy when Bishop’s path to goal was halted and Hackett slotted home the loose ball.
But referee Robert Madden went over to speak to his assistant and after a period of deliberation, decided to disallow the goal.
It left Pompey – those on the pitch, in the dug-out and in the stands – furious and the supporters made their displeasure heard.
Joe Pigott and Harry Jewitt-White were brought on for the closing stages, with the latter making his league debut.
And the Blues had a lifeline on 87 minutes when Hackett beat his man and sent in a cross that was inadvertently turned into his own net by THOMPSON.
That understandably stunned the hosts, but they managed to see out six minutes of added time to claim all three points.
Peterborough (4-4-2): Norris; N.Thompson, Kent, Edwards, Butler; Ward, Kyprianou (Knight 83), Taylor, Mason-Clark; Poku (Burrows 71), Clarke-Harris (c) (Ogbeta 88)
Goals: Clarke-Harris 17, 44 (pen)
Booked: Kyprianou, Clarke-Harris
Subs not used: Blackmore, B.Thompson, Randall, Jones
Pompey (4-3-3): Macey; Swanson, Raggett, Towler, Ogilvie; Tunnicliffe (Jewitt-White 86), Pack (c), Thompson (Hackett 58); Dale (Jacobs 74), Bishop, Scarlett (Pigott 86)
Goals: N.Thompson 87 (og)
Booked: Thompson, Pack
Subs not used: Oluwayemi, Freeman, Hume
Referee: Robert Madley
Attendance: 8,919 (1,561 Pompey fans)