Pompey were brought back down to earth with a bump after losing 2-1 at home to Stevenage.
The visitors seemed content to leave Fratton Park with a point as they soaked up pressure from the Blues.
But Steven Schumacher and Matt Godden both struck in quick succession after the break to leave the home fans stunned.
Michael Smith’s third goal in as many games restored some hope, but Pompey were unable to find a leveller.
Paul Cook unsurprisingly stuck with the same side that won at Luton earlier in the week.
That meant Conor Chaplin had to be content with a place on the bench after returning from suspension, with Smith keeping his place up front.
Pompey made a bright start and a fine cross from Kyle Bennett was just out of reach for Smith running in at the far post.
Bennett went even closer on eight minutes with an effort from the edge of the box that cannoned back off the post.
It was not a half full of clear-cut chances, however, with a solid Boro back-line making it difficult for the Blues to find a way through.
The hosts also had to deal with some rather physical defending and Smith in particular found his shirt being tugged whenever he tried to get on the end of a pass.
But the striker did find some space down the left on 24 minutes, cutting inside and seeing a shot blocked, with Gareth Evans’ follow-up then deflected over.
There was not too much for David Forde to deal with at the other end and when Godden volleyed narrowly wide of the post, the linesman had already raised his flag for offside.
It was still Pompey looking the more likely to break the deadlock, but when Gary Roberts met Carl Baker’s cross he could not get enough power behind his header to test Stevenage keeper Jamie Jones.
There was a surprise at the start of the second half, with Jack Whatmough and Amine Linganzi replacing Christian Burgess and Michael Doyle in a double switch.
It was the Blues who were first to threaten after a ragged opening when Bennett cut in from the left and stung Jones’ fingertips with a fierce shot.
But there was none of the passing fluency that was evident before the break, with the hosts guilty of some sloppy play.
Stevenage were still lacking in ambition, but a foul from Matt Clarke led to Schumacher curling a free-kick narrowly over the bar.
Cook made his last throw of the dice on 68 minutes, trying to inject a fresh attacking impetus by replacing Bennett with Chaplin.
The striker soon had a chance to break the deadlock, but fired over from close range following good work from Smith and Roberts.
But instead it was the visitors who scored against the run of play on 71 minutes, when Pompey failed to clear their lines and Schumacher slotted home.
That proved to be the midfielder’s final kick of the game as he was replaced by Charlie Lee before the game resumed.
It soon got even worse for the Blues, however, as Stevenage regained possession from a miscued Forde clearance and Godden slid the ball through the keeper’s legs.
But some hope was restored 10 minutes from the end when a cross from Evans missed Chaplin, but was lashed home by SMITH at the far post.
Pompey poured forward in search of an equaliser and Roberts hit a low effort narrowly wide on 89 minutes.
But Boro got plenty of men behind the ball to frustrate the Blues and when Whatmough headed against the post in stoppage-time it proved to be the last chance to salvage a point.
Pompey (4-2-3-1): Forde; Evans, Burgess (Whatmough 46), Clarke, Stevens; Rose, Doyle (c) (Linganzi 46); Baker, Roberts, Bennett (Chaplin 68); Smith
Goals: Smith 80
Booked: Clarke, Evans
Subs not used: O’Brien, Naismith, Lalkovic, Hunt
Stevenage (4-4-2): Jones; Henry (c), Wilkinson (Wells 46), Franks, Ntlhe; Cowans, Schumacher (Lee 72), King, Gorman (McKirdy 60); Godden, Kennedy
Goals: Schumacher 71, Godden 75
Booked: Gorman, Wilkinson, King
Subs not used: Day, McAnuff, Tonge, Pett
Referee: Ben Toner
Attendance: 15,987 (277 away fans)