Pompey exited the Carabao Cup after a penalty shoot-out defeat to fellow third tier side Peterborough at Fratton Park.
David Ajiboye’s opener for the visitors was cancelled out after the break by Christian Saydee’s first Blues goal.
Charlie O’Connell was then sent off for Posh, who survived plenty of late pressure to see out the 90 minutes.
And with Abu Kamara and Joe Morrell both missing from the spot, Ajiboye was able to fire them into round three.
John Mousinho made 10 changes from the side that were held to a League One draw at Stevenage at the weekend.
Jack Sparkes was the only player to retain his place in the starting line-up, as both clubs decided to offer minutes to other members of their squads.
There was another chance to impress for 16-year-old academy forward Koby Mottoh, who made up a front three with Saydee and Paddy Lane.
Ben Stevenson, Alex Robertson and Terry Devlin came into the midfield, with Sparkes and Zak Swanson occupying the full-back roles.
The defence was completed by Ryley Towler and stand-in skipper Sean Raggett, while Ryan Schofield was between the sticks.
Pompey started brightly on a damp evening and began to apply plenty of pressure on their opponents.
A low cross from Swanson was just behind Devlin in the box, while the Northern Irish youngster then hit a shot that was just about scrambled over by Fynn Talley.
There were also a series of corners – from Sparkes and Swanson – that would not quite land for a blue-shirted player.
But Posh then created a couple of chances on their own, with Schofield getting down well to deny Ajiboye before a Ricky-Jade Jones effort was deflected narrowly wide.
The action was soon back down the other end and a dangerous drive from Robertson was not held by Talley.
Lane tried his best to tuck away the loose ball, but the keeper recovered to deny him from point-blank range.
And the winger ended up colliding with the post, leading to an anxious few minutes before he was able to continue.
Robertson soon had another attempt, this time hitting a low 25-yard shot that Talley was able to parry to safety.
It was Peterborough who broke the deadlock on 29 minutes, however, after Kai Corbett swung in a corner.
Kabongo Tshimanga’s header was wayward, but Josh Knight kept the ball in play and it was fired home by Ajiboye.
Pompey lost their way after that and they took a while to regain the rhythm they had showcased in the early stages.
Lane did whip in a teasing cross from the right in stoppage-time, although it was just too high for Saydee at the back post.
Half Time: Pompey 0 Peterborough 1
The signs did not immediately look great following the interval, with the hosts producing some sloppy play in possession.
But they found themselves level on 51 minutes after Raggett’s long pass found Lane in space on the right.
He then picked out SAYDEE in the box, with the striker controlling the ball on his chest before hooking it home to open his Blues account.
And things soon got worse for the visitors, as O’Connell – earlier booked for kicking the ball away – fouled Devlin and was dismissed by referee Charles Breakspear.
Mousinho freshened things up with a double switch soon after, as Mottoh and Swanson made way for Kamara and Regan Poole.
And a third change followed for the hosts moments later, with Colby Bishop called upon to replace Robertson.
The striker was then furious when he went down under an Emmanuel Fernandez challenge, only for a foul to be awarded in the visitors’ favour.
Lane had a good chance after Kamara’s low centre had made its way to the far post, but he could only fire over.
Pompey were certainly applying plenty of pressure as they searched for the winning goal that would prevent penalties.
Marlon Pack and Morrell replaced Stevenson and Devlin for the closing stages, with Posh holding on for dear life.
The former almost grabbed a stoppage-time winner after Saydee was fouled, but his low free-kick was turned around the post by Talley.
Full Time: Pompey 1 Peterborough 1
Bishop, Raggett, Pack and Saydee all tucked their spot-kicks away and it looked like Pompey were heading for round three when Schofield kept out Ryan De Havilland’s effort.
But Kamara then sent his penalty over the bar and then, in sudden death, Morrell fired against the post.
It meant that Ajiboye had the chance to win it for Peterborough and he sent Schofield the wrong way to seal their progress.
Pompey (4-3-3): Schofield; Swanson (Poole 62), Raggett (c), Towler, Sparkes; Robertson (Bishop 67), Stevenson (Pack 89), Devlin (Morrell 89); Lane, Saydee, Mottoh (Kamara 62)
Goals: Saydee 51
Booked: Robertson, Stevenson
Subs not used: Norris, Quarm, Ogilvie, Folarin
Peterborough (3-4-3): Talley; Dornelly (Mills 79), Knight (c), Fernandez; Ajiboye, O’Connell, De Havilland, Tomlinson; Jones, Tshimanga (Titchmarsh 84), Corbett
Goals: Ajiboye 29
Sent off: O’Connell (two yellow cards)
Booked: Fernandez
Subs not used: West, Freeman, Toynton, O’Brien-Brady, Holley, Beech, Overton
Referee: Charles Breakspear
Attendance: 6,594 (189 away fans)