Kusini Yengi came off the bench to fire Pompey to a vital victory over League One promotion rivals Peterborough at London Road.
In a breathless contest that often switched from one end to the other, it looked increasingly likely that the spoils would be shared.
But some excellent play from fellow substitute Gavin Whyte allowed Yengi to slot home a goal that sent more than 4,000 travelling Blues fans wild with delight.
John Mousinho made a trio of changes from the side that beat Burton on home soil earlier in the week.
Zak Swanson was handed a first start in almost three months, while Jack Sparkes returned following a minor ankle injury.
There was also a place for Colby Bishop up top, with Joe Rafferty and Yengi dropping to the bench and Paddy Lane absent completely.
Both sides showed their attacking intent in the early stages, as the action quickly switched from one end to the next.
The visitors were causing problems by winning the ball high up the pitch, while their Posh opponents broke with menace.
Marlon Pack was looking as lively as ever and a 30-yard free-kick was spilled by Jed Steer, who was thankful to see Harrison Burrows clear the danger.
Peterborough’s left-back was immediately involved again at the other end of the pitch, drilling narrowly wide.
Abu Kamara was then denied a penalty after going down in the area, while Christian Saydee was dispossessed just as he shaped to shoot.
Ephron Mason-Clark was proving to be the most dangerous player for the hosts and after being picked out on the left, he smashed the ball against the underside of the crossbar.
Both sides were looking more than capable of breaking the deadlock and when Pack swung in another free-kick, Bishop’s header was saved by Steer.
Pompey were forced into a change on 32 minutes, when Saydee went down and was replaced by Yengi.
The chances kept coming and a teasing delivery from Owen Moxon was agonisingly out of reach for Bishop in the box.
A Posh attack soon followed and when Will Norris kept out Mason-Clark’s drive, the loose ball was cleared by Conor Shaughnessy.
It was the visitors who finished the stronger of the sides, though, applying plenty of stoppage-time pressure.
A Sparkes corner was only partially cleared and the ball was sent back into the mixer by Myles Peart-Harris.
Shaughnessy tried to turn it into the net, but Burrows made the block, as did Ronnie Edwards from Bishop’s follow-up attempt.
Half Time: Peterborough 0 Pompey 0
It was still proving to be a tight and tense contest when the action got restarted, with both sides trying to get forward.
There was a series of corners from the home side, but each of them were comfortably dealt with by the Blues.
They did have a scare when Jadel Katongo picked out Malik Mothersille, who flicked the ball narrowly wide.
Peart-Harris collected a neat pass from Moxon at the other end, but the loanee could not keep his effort down.
Kamara then won a corner following some good work on the right and the ball had to be cleared behind when Steer failed to hold Sparkes’ cross.
Mousinho made a second switch just past the hour mark, as Peart-Harris made way for Whyte out wide.
The chances began to dry up slightly, with both sides unable to make the most of promising attacking plays.
But the noisy travelling army grew even louder when their side forged ahead on 77 minutes following a quick break up the pitch.
Whyte laid the ball off for Bishop and then raced up the pitch to collect it back again before switching play to YENGI on the left.
The Australian striker surged past Edwards and then slipped a shot inside Steer’s near post before racing away to celebrate.
Pompey kept pushing forward, though, and after winning the ball inside the Posh box, Yengi could not quite force the ball home again.
Their opponents threw on forward players to try to find a late leveller, but the Blues saw out the closing stages in comfortable fashion.
Peterborough (4-2-3-1): Steer; Katongo (Clarke-Harris 90), Knight, Edwards, Burrows (c) (Fernandez 90+3); Collins, De Havilland; Poku (Ajiboye 90+3), Randall, Mason-Clark; Mothersille (Jones 65)
Booked: Poku
Subs not used: Bilokapic, Crichlow, Olakigbe
Pompey (4-2-3-1): Norris; Swanson, Raggett, Shaughnessy, Sparkes; Moxon, Pack (c); Kamara, Saydee (Yengi 32), Peart-Harris (Whyte 61); Bishop
Goals: Yengi 77
Booked: Sparkes, Pack
Subs not used: Macey, Rafferty, Towler, Evans, Martin
Referee: Alex Chilowicz
Attendance: 12,744 (4,019 Pompey fans)
