Nicke Kabamba netted on his fourth consecutive reserve outing to cap a memorable 48 hours.
The striker popped up in the 80th minute to bundle home the winner against Sheffield United last night.
In a tight game of few goal-scoring opportunities, it was enough to settle the occasion and book Pompey’s reserves into the quarter-finals of the Premier League Cup.
Double delight for Kabamba, who came off the bench at Stevenage on Saturday for his Football League debut.
Irrespective of that defeat, it was still a moment to treasure for the January arrival from Hampton & Richmond.
Kabamba has also now netted in all four of his reserve appearances, also scoring against Norwich, Wolves and Everton.
And his latest goal helped Mikey Harris’ side into the final eight of a competition they entered in the preliminary stages.
Pompey named Conor Chaplin in their starting line-up, along with fellow substitutes from Stevenage in Amine Linganzi, Jamal Lowe, Kabamba, Drew Talbot and Liam O’Brien.
There was also another start for promising Academy youngster Theo Widdrington, who operated at centre-half.
Meanwhile, Adam May took time out from his Sutton United loan spell to skipper Mikey Harris’ side.
When the match got underway, there was little to admire about the first half as both sides struggled to create.
There was a glaring absence of any goalmouth action, with it taking 37 minutes before the hosts offered something.
Brandon Haunstrip sent in a deep cross from the left which was met by a towering Kabamba header, but the Blades keeper was its equal.
Then May fed Chaplin inside the left-hand channel of the penalty area, who put his right-foot shot straight at Josh Perryman.
That was the sum of Pompey’s parts during a lacklustre opening 45 minutes.
At the start of the second half, Horatio Hirst drove in a shot from the edge of the area, but O’Brien blocked it.
For Pompey, the game burst into life on 53 minutes as they almost broke the deadlock.
Firstly Lowe burst down the left into the penalty area and saw his left-foot shot parried by keeper Josh Perryman.
Chaplin attempted to crash a follow-up into an unguarded net but a flying United body partially blocked it to go out for a corner.
The Blues did, though, break the deadlock on 80 minutes through Kabamba.
Lowe drilled in a left-foot shot and when Perryman parried the ball ricocheted off the lurking striker to bundle into the net.
Kabamba should have out the game beyond doubt in the 90th minute, but somehow failed to tap home Chaplin’s pass from close range.
Not that it mattered, as Pompey progress into the competition’s quarter-finals.
Pompey: O’Brien, Davies, Talbot, Widdrington, Haunstrup, Oxlade-Chamberlain (61 mins Bedford), May, Linganzi, Lowe, Kabamba, Chaplin.
Subs Not Used; Bass, Bradbury, Wakley, Woodward.
Sheffield United: Perryman, Bennett, Charlesworth, Harrington, Kelly, Cantrill, Smith, Slater, Wright, Gilmour, Hirst (76 mins Smith).
Subs Not Used; Warhurst, Ford, Ferguson, Greaves.