Pompey Academy were involved in another high-scoring Youth Alliance League contest, but were beaten 6-2 by Oxford at Furze Lane.
It was the second time the sides had met in the space of a few days, with United having triumphed on penalties in a cup clash earlier in the week.
Will Owens took advantage of the strong winds to put the visitors ahead, only for the Blues to hit back when Harry Jewitt-White drove forward and fired home from the edge of the area.
But Jack Gardner and Adam Smith both netted for Oxford and Jewitt-White’s red card for dissent meant it was going to be an uphill challenge after the interval.
Callum Crook increased Pompey’s deficit, although they did pull one back when Dan Gifford was fouled in the box and Charlie Bell converted the penalty.
But United made the most of their numerical advantage, with a fluke effort from Owens and Dan Cadman’s own-goal wrapping things up.
Professional development phase coach Liam Daish (pictured) said: “We started the game very well and were disappointed to concede from our opponents’ very first attack.
“The conditions were terrible and I think Leon misjudged things in the strong winds, as the ball swirled into the net.
"We still created enough opportunities to get something from the game – even when we were down to 10 men – but our discipline and decision making let us down at times."
“We got back into the game, though, and had chances to go ahead, but made the wrong decisions and conceded some poor goals by our standards.
“Harry was then sent off just before the break. It’s something he’s going to have to learn from and it left us a man down for the whole of the second half.
“There was another freak goal that Oxford scored due to the conditions and also a mix-up between Dan and the keeper, so it was a tough day for us.
“We still created enough opportunities to get something from the game – even when we were down to 10 men – but our discipline and decision making let us down at times.”
Pompey will look to bounce back from their double Oxford disappointment when they take on Didcot at Fratton Park in the first round of the FA Youth Cup on Tuesday evening.
Daish: “It should give the boys a bit of a lift to play under the lights on such a good stage and with an excellent pitch.
“We’ll have to wait and see what’s going to happen during lockdown, but that’s something out of our control.
“All we can do is try to produce a really positive performance and do our best to get through to the next round.”
Pompey: Pitman (Manderson 59); Setters (Gifford 64), Kaba, Cadman, Rew; Jewitt-White, Bell; Bridgman (Hughes 64), Lee, Brook; Anderson (Dawson 64)
Click here to see the latest Youth Alliance League table and Pompey appearance stats.