Pompey Academy suffered a narrow defeat to Oxford at Furze Lane in Merit League One.
The only goal of the game came midway through the first half, as Josh Anifowose sliced an effort into the bottom corner.
Pressure started to be applied at the other end as the Blues sought an equaliser and Damien Niemczycki was called into action to save an attempt from Harrison Brook (pictured).
Taylor Seymour them made a decent stop to prevent Callum Crook from doubling United’s lead early in the second half.
The contest became more open as the hosts committed bodies forward and they almost grabbed a leveller, with Brook crashing a shot against the underside of the crossbar.
Professional development phase coach Liam Daish said: “It was a tough game in really difficult windy conditions down at the university.
“Oxford are a strong side defensively. They don’t mess about, are quite physical and maybe adapted that little bit better.
"It was a hard game, but I can’t really fault the boys for their effort or attitude. We just need a bit more guile."
“We controlled the second half, but just lacked that cutting edge. It was a match where both back-lines finished on top and we just needed a bit more quality in the final third.
“There were chances and we hit the woodwork, so it was a little bit frustrating that we were unable to break them down.
“Had we got the equaliser, then you feel we could have gone on to have won, but credit to Oxford because they put bodies on the line to see us off.
“It was a hard game, but I can’t really fault the boys for their effort or attitude. We just needed a bit more guile.”
Pompey: Seymour; Bruce (Hughes 41), Rew, Robb, Kelly; Storey, Bell; Teggart (Anderson 60), Lee (Jewitt-White 46), Brook; Stanley