Pompey missed out on securing top spot in Southern Group A of the Papa John’s Trophy after suffering a narrow defeat to West Ham U21s at Fratton Park.
Kai Corbett netted the only goal of the contest against an extremely youthful home side, with six teenagers making their maiden senior outing for the club.
Both times had already qualified for the knockout stages, although a first loss in the competition for almost three years ensured the Blues will be on the road in the next round.
Kenny Jackett made 11 changes from the side that won at Ipswich to progress in the FA Cup at the weekend.
He handed debuts to third year scholars Harry Kavanagh and Alfie Stanley, as well as academy midfielder Charlie Bell.
Jordy Hiwula was given a first start for the club, joining fellow senior professionals Paul Downing – who skippered the team – James Bolton, Bryn Morris and Alex Bass.
Haji Mnoga was provided with another chance to impress, while Eoin Teggart and Harvey Rew completed the line-up following their appearances in the same competition last season.
Nobody on the bench had featured in a senior game for Pompey, as Duncan Turnbull joined a host of players from the under-18 side.
But the young Blues found themselves behind after just four minutes, as Emmanuel Longelo cut the ball back for Corbett to squeeze home a shot.
The hosts took a while to find their feet and there were a few passes going astray in the opening stages.
But they gradually began to settle and play some nice possession football, without really being able to test Nathan Trott.
The Hammers keeper was called into action for the first time on 19 minutes, but not overly troubled by Kavanagh’s header from a Teggart delivery.
Stanley and Hiwula then did well to steal possession and launch an attack, with Alfie Lewis’ foul on the latter earning him a booking.
Pompey were the stronger side as the interval neared and they almost found an equaliser before the whistle blew.
Hiwula showed some nifty footwork inside the box, but his low drive was saved by Trott’s feet at the keeper’s near post.
Half Time: Pompey 0 West Ham U21 1
There did not look much on following the restart when Mnoga overhit his pass over the top towards Kavanagh.
But Longelo made a mess of his pass back to Trott and the keeper did well to get there before the hosts could capitalise.
Oladapo Afolayan then hit something that was between a cross and a shot at the other end, with the effort ultimately flashing past the post.
The same player then struck a fierce shot towards the target, only for Bolton to bravely fling himself into the path of the ball.
Pompey were trying to launch some threatening attacks and a delivery from Teggart drifted towards goal, with Trott having to claim it underneath his own bar.
Jackett made a double switch on 66 minutes, as Harry Jewitt-White and Harrison Brook – aged 16 and 17 respectively – replaced Stanley and Brook to become the latest debutants.
The hosts were playing some really nice stuff, with Rew and Teggart linking up particularly well down the left, but were not making too much of an impact in the final third.
Seok Jae Lee was also handed his Blues bow in the closing stages, as Bell made his way from the pitch.
But it was the more senior Bolton who had a chance to grab a late leveller, stretching to get on the end of Teggart’s cross and hooking the ball narrowly over.
And the Hammers almost added a second goal just before the end, hitting a ferocious strike that cannoned back off the crossbar.
Pompey (4-4-2): Bass; Mnoga, Bolton, Downing (c), Rew; Kavanagh (Brook 66), Bell (Lee 79), Morris, Teggart; Stanley (Jewitt-White 66), Hiwula
Booked: Mnoga, Downing
Subs not used: Turnbull, Setters, Hughes, Bridgman
West Ham U21 (3-4-3): Trott; Okotcha, Johnson (c), Greenidge; Rosa (Forbes 80), Appiah-Forson, Lewis, Longelo (Chesters 72); Corbett, Afolayan, Swyer (Adarkwa 83)
Goals: Corbett 4
Subs not used: Anang, Fevrier, Potts, Woods
Referee: Kevin Johnson