Bradley Lethbridge scored his first senior Pompey goal in a draw with Oxford in the Leasing.com Trophy.
The striker cancelled out Matty Taylor’s opener at a sparsely-populated Kassam Stadium in Southern Group B.
Anton Walkes then looked to have won it for the youthful visitors with a late strike, only for Rob Dickie to level in stoppage-time.
But the night ended on a happy note for the Blues, as they won 5-4 on penalties to claim a bonus point and secure top spot, guaranteeing a home draw in the second round.
Kenny Jackett changed the entire side that won at Doncaster in the league at the weekend, with five teenagers getting a chance to impress.
Harvey Rew was handed his debut a week after signing a first professional contract and was joined in the starting line-up by Joe Hancott, Josh Flint, Leon Maloney and Lethbridge.
Paul Downing skippered a defence that also included Brandon Haunstrup and Walkes, with Alex Bass between the sticks.
Ryan Williams – who previously had a spell on loan with the hosts – and Andy Cannon completed the team, while there were plenty of experienced reinforcements on the bench.
Pompey’s young side started brightly and enjoyed plenty of possession in the opening stages, without being able to test former Fratton keeper Simon Eastwood.
They had a chance to break the deadlock when Cannon and Williams combined to tee-up Lethbridge, but he screwed wide from 10 yards.
And the more experienced hosts soon started to take control, with Rob Hall dragging a shot wide before Bass denied Taylor at his near post.
Rew was perhaps unfortunate to be shown a yellow card on 19 minutes for a challenge that referee Will Finnie deemed excessive.
United then broke the deadlock soon after, as Taylor latched onto Shandon Baptiste’s threaded pass and slotted home.
And the same player should have immediately doubled the lead, only to see his first effort saved by Bass and the second fly off target.
Pompey picked up after that nervy period, though, and levelled on 33 minutes thanks to a first senior goal for Lethbridge.
It started when Cannon stole possession and cut the ball back for Flint, whose effort was deflected into the striker’s path.
The angle was tight for LETHBRIDGE, but he got enough power on his shot for the ball to squirm under Eastwood and over the line.
It was United who finished the half on top, however, and a fierce drive from Cameron Brannagan had to be parried down by Bass and gathered at the second attempt.
And the keeper made an even better save just before the break, acrobatically tipping over Taylor’s firm header from a Josh Ruffels delivery.
Half Time: Oxford 1 Pompey 1
Jackett made a double switch at the start of the second half, with Flint and Rew replaced by Gareth Evans and Ben Close.
It was Cannon who had the first shot following the restart, but quickly saw four yellow shirts around him and could only fire wide.
An angled effort from Maloney was then comfortably dealt with by Eastwood in a low-key opening to proceedings.
Then, after Hall had an effort deflected wide at the other end, Dickie diverted a corner straight at Bass.
Jackett made his final switch just past the hour mark, with goalscorer Lethbridge withdrawn and Saturday’s hero Ellis Harrison on in his place.
But it was United on the attack and they had two decent chances to retake the lead in quick succession.
First, Bass came off his line to narrow Taylor’s angle and make a smart stop after the striker had been sent through on goal.
Another header from Dickie was then blocked on the line by Williams, with Finnie waving away handball appeals.
Close has scored a couple of spectacular strikes already this season and tried to add a third when he hit a dipping ball cleanly on the volley, but it lacked the power to overly trouble Eastwood.
Evans then had a great chance after being sent through on goal by Maloney, only to lose his balance at the crucial moment, allowing his shot to be saved.
But Bass was keeping the Blues in the contest and he made another fine save to repel Brannagan’s fierce angled drive on 75 minutes.
And Walkes was also called into action moments later to block a powerful goal-bound effort from substitute Dan Agyei.
Pompey might have capitalised on a quick break, but the onrushing Williams could not quite reach Harrison’s ball across the box.
And they did go ahead on 85 minutes after WALKES raced on to a fine pass down the right from the lively Cannon.
Eastwood came off his line, but the right-back just about got there first and was able to send the ball rolling over the line.
But Oxford managed to draw level in stoppage-time when a free-kick was launched into a crowded box and Dickie spun before finding the bottom corner.
Full Time: Oxford 2 Pompey 2
A penalty shoot-out was then held to decide who would take the bonus point, with Evans, Harrison, Williams and Downing all scoring.
But Close sent his attempt high over the bar to hand the advantage to the hosts, only for Ruffels’ shot to be brilliantly saved by Bass.
Walkes and Agyei both failed to find the target in sudden death and after Haunstrup rolled home his penalty, Sam Long blazed high over the bar.
Oxford (4-1-4-1): Eastwood; Long, Dickie, Moore, Ruffels (c); Gorrin (Sole 86); Hall, Brannagan, Baptiste (Lopes 73), Sykes; Taylor (Agyei 69)
Goals: Taylor 21, Dickie 90+1
Subs not used: Stevens, Jones, Cadden, Fosu
Pompey (4-2-3-1): Bass; Walkes, Downing (c), Haunstrup, Hancott; Cannon, Rew (Close 46); Williams, Flint (Evans 46), Maloney; Lethbridge (Harrison 63)
Goals: Lethbridge 33, Walkes 85
Booked: Rew, Haunstrup
Subs not used: McGee, Burgess, Raggett, Harness
Referee: Will Finnie
Attendance: 1,548 (274 away fans)