Matt Clarke struck right at the death as Pompey beat Burton to keep their automatic promotion quest on track.
A fine strike from Ben Close – just days after the midfielder had penned a new contract – gave them an interval lead.
But the Brewers hit back through Liam Boyce early in the second half and it looked like a stalemate was on the cards.
There was time for a late dose of drama, however, as Clarke fired home following a stoppage-time bundle to seal victory.
Kenny Jackett made one change from the side that eased past Rochdale on home soil the previous week.
Gareth Evans came on to score at Fratton Park and was rewarded with a starting berth, as Ronan Curtis dropped to the bench.
He was joined by a fit-again Viv Solomon-Otabor following the winger’s return to action for the reserves on Tuesday.
There was also a switch of system for the Blues, who began in a 4-3-3 formation, with Jamal Lowe and skipper Brett Pitman flanking Oli Hawkins.
The travelling fans were in fine voice as they enjoyed the Burton sunshine, with plenty packed into the terrace behind the goal and seated on one side.
Their team made a bright start and won a couple of early corners, the first of which was taken by Lee Brown and glanced wide by Clarke at the near post.
Evans then sent one in from the other end, but it was just too high for Tom Naylor – appearing against his former club – at the back post.
It was certainly not one-way traffic, though, and the Brewers caused plenty of problems before the break, without really testing Craig MacGillivray.
Jamie Allen went closest to opening the scoring and drilled a fierce low effort inches wide on 16 minutes.
Stephen Bywater had to come quickly off his line at the other end and just about beat Evans to Close’s delivery.
And it was the latter who broke the deadlock on 31 minutes, playing a key role in the build-up before finishing the move in style.
The midfielder’s initial cross was headed clear, but Nathan Thompson picked the ball back up and laid it off for CLOSE to bend home from the edge of the box.
The visitors charged forward again moments later and Bywater did just enough to stop Lowe slipping a pass across to Hawkins.
Burton looked dangerous themselves, though, and caused an almighty scramble in the Blues box before the ball was eventually hacked to safety.
And there was a repeat just before the break, with Clarke and Evans forced to make blocks inside the area.
There was still time for Pompey to have another chance, however, and Pitman bent a shot narrowly over after being teed-up by Hawkins.
Half Time: Burton 0 Pompey 1
The second half was not even two minutes old when the hosts found themselves level, as Boyce converted Colin Daniel’s fine cross with a firm header from close range.
Lucas Akins soon had another chance for the Brewers, although his effort forced a regulation save from MacGillivray.
Pompey tried to hit back and when Christian Burgess met a free-kick from Evans, his header had to be tipped over by Bywater.
And MacGillivray then did well at the other end, getting down low to his left to push behind a low drive from Stephen Quinn.
Jackett made a double switch just past the hour mark, with Hawkins and Pitman making way for Omar Bogle and Bryn Morris.
But Burton were still causing problems as the second half progressed and forcing their opponents into some sloppy play.
There was some fine bits of individual defending from the Blues, though, including a headed clearance from Burgess and a fine tackle on Scott Fraser by Thompson.
Bogle did have an opportunity for the Blues when he found space to advance down the left, but a weak effort was easily dealt with by Bywater.
And a better opening was wasted moments later, as Morris skied the ball over after Brown had laid it off for him.
Jackett made a final throw of the dice on 77 minutes, with Curtis replacing Evans to inject some attacking impetus.
The newcomer soon did well to beat his man and get into the box, but Lowe was unable to get a shot away.
Both sides were trying their best to grab a late leveller as time ticked away, although it looked like heading for a draw.
Then, deep into stoppage-time, Curtis' cross caused a melee in the Burton box and Thompson somehow managed to keep the play alive.
The ball fell for CLARKE, who slotted it into the net to spark scenes of delirium among the travelling support.
Burton desperately appealed for both offside and a handball, but their cries fell on deaf ears and referee Craig Hicks blew the final whistle soon after.
Burton (4-3-3): Bywater; Brayford (c), McFadzean, Wallace, Daniel; Allen, Quinn (Fox 81), Fraser (Hutchinson 85); Akins, Boyce, Harness (Miller 85)
Goals: Boyce 47
Subs not used: Hawkins, McCrory, Bradley, Clarke
Pompey (4-3-3): MacGillivray; Thompson, Burgess, Clarke, Brown; Close, Naylor, Evans (Curtis 77); Lowe, Hawkins (Bogle 61), Pitman (c) (Morris 61)
Goals: Close 31, Clarke 90+2
Booked: Close, Bogle
Subs not used: Bass, Walkes, Haunstrup, Solomon-Otabor
Referee: Craig Hicks
Attendance: 4,034 (1,290 Pompey fans)