Pompey continued their impressive away record by battling from behind to claim all three points in an eventful game at Cambridge.
Robbie Simpson controversially fired the hosts ahead early on and that led to frayed tempers which boiled over at the break.
United captain Mark Roberts were dismissed following a scuffle and the Blues made good use of their numerical advantage.
They did the damage with two goals in under two minutes, with Leon Legge inadvertently beating his own keeper before Matt Tubbs fired Pompey ahead.
And it was Tubbs who netted again just before the end to make sure that the visitors returned home with a maximum haul, although Gareth Evans soured the mood slightly when he was sent off late on.
Paul Cook stuck with the same side that drew with Yeovil at Fratton Park the previous week.
The hosts named Sam Beasant – son of former Pompey stopper Dave – between the sticks in their starting line-up.
And Gary Roberts tried to catch him out straight from kick-off with a shot from inside his own half that the keeper was wise to.
Aaron McCarey then had come off his line at the other end to punch clear a dangerous Cambridge cross.
But the hosts did take the lead on four minutes – although the Blues were furious that the goal was allowed to stand.
A flag was raised when Simpson raced onto Luke Berry’s pass and slotted home, but – after speaking with his assistant – referee Sebastian Stockbridge ruled in favour of the hosts.
Pompey soon settled into their usual patient passing game and a number of precise crosses were causing United problems, with Ben Davies a particular threat.
But they were unable to make that pressure count by testing Beasant, who had no more than a couple of half-chances from distance to deal with.
The cheers from the travelling supporters were instead of the ironic variety whenever an offside decision went their way.
Roberts was doing his best to find a way through and also tried his luck with a 35-yard pile-driver that fizzed over the crossbar.
He created a promising opportunity by dispossessing Elliot Omozusi on 32 minutes, only for Evans to waste it by firing wildly across the face of goal.
But there were to be no more major moments of drama until after the half-time whistle was blown.
A scuffle then broke out between the two teams and assistant manager Leam Richardson came onto the pitch to usher his players away and down the tunnel.
In the melee, Cambridge skipper Roberts pushed McCarey to the ground and was sent off by Stockbridge, while Richardson was ordered to watch the rest of the game from the stands.
Cambridge United 1
It was the 10-man hosts who were first to threaten when the game restarted and McCarey was down quickly to smother Ryan Donaldson’s effort.
Pompey immediately had a chance down at the other end, but Kyle Bennett could not guide Enda Stevens’ cross on target.
Cambridge were sitting increasingly deeper as the Blues tried to find a way through and Bennett forced a decent stop from Beasant just past the hour mark.
Cook made his first switch on 66 minutes and it was certainly an unusual move, with Adam Webster replacing Matt Clarke and Burgess moving up front.
And, within the space of just a few seconds, Pompey had scored twice to turn the game completely on its head.
The first came when United defender LEGGE, under pressure from Tubbs, steered a decent Evans cross into his own net.
And the large away following were in dreamland moments later, as TUBBS showed his poacher’s instinct to convert Roberts’ pass from close range.
Burgess immediately dropped back into defence and a second substitution followed on 75 minutes, with Kal Naismith replacing Bennett.
Pompey did not have too much to deal with at the other end as they looked to see out the rest of the game.
And they even managed to grab a third goal in fortunate fashion late on, with a clearance hitting Naismith and rebounding off the bar for TUBBS to head home.
The only sour note was a stoppage-time dismissal for Evans, who was shown a straight red card for a late challenge on Omozusi.
Cambridge (4-4-2): Beasant; Omozusi, Legge, Roberts (c), Demetriou; Donaldson (Newton 73), Berry, Hughes, Dunk (Morrissey 83); Corr, Simpson (Taft 46)
Goals: Simpson 4
Sent off: Roberts
Booked: Corr, Newton
Subs not used: Taylor, Hughes, Gaffney, Akintunde
Pompey (4-2-3-1): McCarey; Davies, Burgess, Clarke (Webster 65), Stevens; Barton (Atangana 89), Doyle (c); Evans, Roberts, Bennett (Naismith 75); Tubbs
Goals: Legge 67 (og), Tubbs 68, 87
Sent off: Evans
Subs not used: Bass, Haunstrup, Hollands, Chaplin
Referee: Sebastian Stockbridge
Attendance: 6,607 (1,866 Pompey fans)