Pompey lost 2-1 at Oxford to ensure they suffered back-to-back defeats for the first time this season.
Goals from Cameron Brannagan and James Henry meant the hosts enjoyed a comfortable lead at the interval.
The Blues were well below par, but Brett Pitman came on to spark hopes of a comeback with a fine finish.
Those 1,803 travelling fans were not given a result their backing deserved, however, as United held on for the points.
Kenny Jackett made three changes from the side that were beaten by Blackpool on home soil the previous week.
One was enforced, with Andre Green being recalled from his loan spell by Aston Villa, while Dion Donohue and Pitman dropped to the bench.
A fit-again Lee Brown returned at left-back, Andy Cannon was handed his full debut and David Wheeler also got a rare league start.
It meant that Jamal Lowe led the line for Pompey and skipper Gareth Evans moved into a more advanced role.
Matt Clarke’s appearance in the side brought up 150 appearances for the defender, but new signing Bryn Morris had to be content with a spot among the substitutes.
The visitors started brightly and won an early corner that was dealt with, while a similar set-piece at the other end was nodded wide by Rob Dickie.
A deep free-kick from Brown then caused problems in the United defence, who were eventually able to scramble the ball to safety.
There were not an abundance of opportunities, with both sides showcasing attacking intent, but unable to produce any meaningful shots.
A couple of set-pieces were comfortably dealt with by Craig MacGillivray, while Evans’ curler from the edge of the box was not too far off target.
But it was the hosts who broke the deadlock midway through the first half, although it might have been the Blues celebrating a goal.
Lowe could not quite charge the ball down following a slip from former Fratton keeper Simon Eastwood.
Instead his long clearance eventually made its way to Brannagan, who drilled a fine low shot right into the bottom corner.
Oxford might have doubled their lead soon after, as a corner led to a spot of pinball inside the Pompey box.
The visitors won a couple of their own soon after and Eastwood dropped the second, but grabbed the ball at the second attempt.
It became a niggly affair as the interval approached, but United were able to double their advantage on the stroke of half-time.
Jamie Mackie was unable to get a shot away and instead played the ball across for an unmarked Henry to lash home.
Half Time: Oxford 2 Pompey 0
There were no substitutions for the Blues at the break, although Wheeler and Lowe swapped positions.
And the former had a chance to reduce the deficit moments after the restart, meeting Evans’ free-kick and sending a header just off target.
But it was still Oxford looking more likely to add to the scoring and an effort from Mackie flew narrowly over.
Then, after a Lowe strike had been comfortably dealt with by Eastwood, the hosts had two more chances to put the result beyond doubt.
First, a cross from Jordan Graham was punched away by MacGillivray and Jamie Hanson’s follow-up was deflected off target by Clarke.
Gavin Whyte’s corner was then palmed clear by the Blues stopper and this time Brannagan’s attempt flew into the side netting.
With the visitors struggling to find any kind of fluency, Jackett made a double change just before the hour mark.
Wheeler and Evans were the men to make way, with Pitman and Louis Dennis coming on to replace them.
And it proved to be an inspired decision, as the duo combined to reduce Pompey’s deficit on 64 minutes.
Dennis provided the assist, crossing the ball for PITMAN to convert with an acrobatic overhead effort that left Eastwood helpless.
The Blues were struggling to build on that goal, however, and Cannon made way for Donohue in a final throw of the dice.
One patient move did lead to a chance on 81 minutes, although Lowe could not adjust his body enough to guide Brown’s delivery on target.
And despite producing plenty of effort in the closing stages, Oxford remained firm at the back to secure victory.
Oxford (4-2-3-1): Eastwood; Hanson, Nelson, Dickie, Ruffels; Mousinho (c), Brannagan; Whyte (Kashi 86), Henry, Graham (Browne 76); Mackie
Goals: Brannagan 25, Henry 45
Subs not used: Stevens, Long, Garbutt, Sykes, Carruthers
Pompey (4-2-3-1): MacGillivray; Walkes, Whatmough, Clarke, Brown; Cannon (Donohue 76), Naylor; Wheeler (Pitman 59), Evans (c) (Dennis 59), Curtis; Lowe
Goals: Pitman 64
Booked: Lowe, Whatmough, Naylor
Subs not used: Bass, Burgess, Haunstrup, Morris
Referee: Christopher Sarginson
Attendance: 8,202 (1,803 Pompey fans)