The Blues found themselves trailing Reading by two goals after 27 minutes following strikes from Lewis Wing and Charlie Savage.
Yet John Mousinho’s men have demonstrated their fighting spirit time and time again, only on this occasion it was even more remarkable.
It was 2-2 by half-time, with Tino Anjorin and Colby Bishop restoring parity before the first half was even over.
Then Terry Devlin, on his maiden Football League start, volleyed the winner on 58 minutes to cap a stunning fightback.
It leaves the Blues six points clear at the top of League One after 15 matches – and still unbeaten.
Mousinho had sprung another selection shock by handing Devlin his maiden Football League start.
Replacing Christian Saydee in an attacking midfield role behind, it represented one of our changes to the side which collected a goalless draw at Cambridge United.
Elsewhere, Tino Anjorin was recalled in place of Abu Kamara on the left flank, while Jack Sparkes came in for the injured Connor Ogilvie.
Completing the changes was Joe Rafferty’s return to right-back, with Zak Swanson dropping to the bench.
Meanwhile, Gavin Whyte and Sean Raggett were among the substitutes having recovered from illness, with Ryley Towler and Ben Stevenson not included in the 18-man squad.
The match was delayed after three minutes after a flurry of tennis balls were thrown onto the pitch, primarily landing around Pompey’s penalty area.
The deadlock was broken on 24 when Sam Smith’s cross from the left was headed out by Conor Shaughnessy as far as Wing, who took a touch before rifling a left-footed shot high into the net.
It was 2-0 just three minutes later when Andy Yiadom beat Anjorin to deliver a cross from the right which eventually fell to Savage, who lashed it home left-footed.
Moments later, David Button produced an excellent save to keep out a Shaughnessy’s header from Jack Sparkes’ left-wing free-kick.
Yet Pompey did reduce the deficit on 33 minutes when Paddy Lane’s cross from the right was met with a lovely controlled right-footed volley from Anjorin just inside the box to make it 2-1.
Reading’s Smith then went close to restoring their two-goal advantage when he rounded Will Norris only to fire over an open goal from outside the box.
The Blues, however, levelled in the ninth minute of time added-on when Devlin’s shot was charged down and fell to Lane, who calmly squared it for Bishop to tap home.
Mousinho’s men started strongly after the break and Anjorin’s overhead kick drew another fine save from the busy Button.
However, the visitors took the lead for the first time in the match on 57 minutes through Devlin.
Sparkes’ cross from the left was cleverly headed back across goal by Bishop and there was the Irishman with a wonderful right-footed volley into the roof of the net to make it 3-2.
That was enough to secure another victory for the league leaders who are now six points clear of second-placed Oxford.