Bradley Lethbridge bagged a brace as Pompey Academy battled from behind to beat Oxford 2-1 at Furze Lane.
The hosts made a sluggish start to proceedings and their opponents capitalised on that to lead at the interval.
But the Blues were much stronger in the second half and Lethbridge (pictured) latched on to Oscar Johnston’s threaded pass to level.
Lethbridge then pounced to grab the winner when Leon Maloney crossed and the United keeper failed to hold Matt Mayes’ effort.
Academy manager Mark Kelly said: “We were off the pace in the first half and took time to settle into the contest.
“We had a couple of early chances to score, but Oxford were then on the front foot and managed to forge ahead.
"We told the boys, in no uncertain terms, to up the tempo and that’s exactly what they did."
“They had opportunities to extend their lead after that, with Jack Collins making some good saves to make sure we only trailed by one goal.
“At half-time we told the boys, in no uncertain terms, to up the tempo and that’s exactly what they did. We soon started to take control.
“We saw some encouraging performances, with Matt Casey solid at the back, Leon looking bright and Brad getting his two goals.
“With a lot of reserve fixtures to fit in, we’re trying to split the game time. A few under-16s were involved, as well as the first year scholars who haven’t been playing as much.
“It’s another tough week ahead, with a Premier League Cup match at Everton on Tuesday, so it’s a bit of a balancing act.”
Pompey: Collins; Kavanagh, Dandy (Mnoga 46), Casey, Hancott; Johnston, Flint (Mayes 60), Read (Brooks 52), Whiting; Maloney, Lethbridge