Dane Scarlett and Zak Swanson were on target as Pompey beat Ipswich and booked their place in the last 16 of the Papa Johns Trophy.
They had earlier soaked up a lot of pressure before Scarlett’s neat finish stunned their hosts just before the interval at Portman Road.
Both sides had chances to add to the scoring in the second half, but the Blues made sure of their progress when substitute Swanson slotted home in stoppage-time.
Danny Cowley made seven changes from the side that drew with Derby in the league on Friday night.
Scarlett, Owen Dale, Jay Mingi and Connor Ogilvie were the only players to keep their spot in the starting line-up.
Josh Oluwayemi was selected between the sticks, with Denver Hume, Kieron Freeman and Michael Morrison named in defence, the latter wearing the captain’s armband.
The others to return to the team were Ryan Tunnicliffe, Ronan Curtis and Reeco Hackett, while an experienced bench included leading marksman Colby Bishop.
Marcus Harness was selected by the hosts to face his former club, but the injured Conor Chaplin and cup-tied Gassan Ahadme were both absent.
Ipswich made the brighter start and almost took an early lead when Greg Leigh got on the end of Sam Morsy’s delivery, only for Oluwayemi to save with his legs.
Kayden Jackson then sent a shot over the bar from inside the box after being teed-up by Kane Vincent-Young.
And the well-placed Panutche Camara could not make a clean enough connection with Leigh’s cross, sending his header bouncing down into the turf and wide.
But the Blues suddenly found their rhythm towards the end of the first half and began to trouble the Tractor Boys defence.
Dale was a lively presence on the right and sent an effort flying into the arms of Václav Hladký after collecting an incisive Curtis pass.
And he was involved again when the deadlock was broken on 43 minutes, advancing forward before eventually being unable to get past George Edmundson.
The ball flew into the path of SCARLETT, however, and his precise low shot clipped the inside of the post on its way into the net.
The Blues amazingly then had two opportunities to extend their lead in stoppage-time, with Dale’s effort beaten away by Hladký following good work by Hackett.
And Dale soon took advantage of some sloppy play from the hosts to find Scarlett, but the loanee could not quite manage to lift his shot over Hladký.
Half Time: Ipswich 0 Pompey 1
The home support grew increasingly frustrated by some more questionable defending once the action restarted.
And they might have been punished by some neat passing play from Pompey soon after, but Dale could only curl a shot into the hands of Hladký.
Freeman then combined well with Hackett down the right and the latter’s cross was met by a Morrison header that landed on the roof of the net.
Pompey collected a couple of silly bookings around the hour mark, with Morrison punished for taking too long over a free-kick and Dale for kicking the ball away after conceding a foul.
Ipswich were beginning to apply some pressure, but Scarlett tried to hit back with an audacious attempt from just inside the opposition half that a nervous Hladký was able to deal with.
Cowley freshened things up with a quadruple change on 67 minutes, as Clark Robertson, Sean Raggett, Swanson and Bishop replaced Ogilvie, Mingi, Freeman and Hackett.
It saw a switch to a three-man defence, with Dale moving to right wing-back and Swanson taking up a midfield role.
The Tractor Boys were doing their best to find a leveller, but Oluwayemi was being well protected by his outfield colleagues.
Pompey’s final change came on 79 minutes, as Scarlett – who appeared to have picked up an earlier knock – made way for Josh Koroma.
The newcomer might have wrapped things up after beating Hladký in a race for the ball, but Edmundson came to his keeper’s rescue.
But the Blues were remaining resolute in their efforts to see out the closing stages, as the Tractor Boys applied pressure.
The victory was assured right at the death, however, as the visitors broke up the pitch and Koroma played in SWANSON to slot home.
Ipswich (3-4-2-1): Hladký; Woolfenden (Burgess 62), Keogh, Edmundson; Vincent-Young, Morsy (c), Humphreys, Leigh (Burns 62); Camara (Armin 75), Harness (Siziba 24); Jackson
Booked: Morsy
Subs not used: Coleman, Agbaje, Ladapo
Pompey (4-2-3-1): Oluwayemi; Freeman (Swanson 67), Morrison (c), Ogilvie (Robertson 67), Hume; Tunnicliffe, Mingi (Raggett 67); Dale, Hackett (Bishop 67), Curtis; Scarlett (Koroma 79)
Goals: Scarlett 43, Swanson 90+4
Booked: Morrison, Dale, Koroma
Subs not used: Griffiths, Gifford
Referee: Tom Nield
Attendance: 5,686 (245 Pompey fans)