Conor Chaplin’s first-half strike earned Pompey Reserves a 1-0 win against Southend and a place in the Premier League Cup group stage.
The Academy product stabbed home from close range in the seventh minuteto secure his side a hard-fought victory at Westleigh Park.
Chaplin also crashed a penalty against the bar deep into stoppage time.
Pompey went with a 4-2-3-1 formation with Nathan Thompson – returning from a foot injury he picked up on the first day of pre-season – featuring at centre-back and wore the captain’s armband.
Christian Oxlade-Chamberlain was surprisingly deployed at right-back while triallist Mohamed-Labib Maouche operated in a holding midfield role.
Southend had the first sight on goal in the second minute when Michael Klass broke down the left, but his effort was stopped by Alex Bass.
Thompson almost made a dream start wearing the blue shirt for the first time.
He met Theo Widdrington’s corner in the fourth minute but the Blues’ skipper for the night was unable to divert his header on target.
The breakthrough came three minutes later through another corner.
Widdrington fizzled the ball into the box and, after a goalmouth scramble, Chaplin poked home to give his side the lead.
Pompey almost doubled their advantage in the 16th minute.
Chaplin rolled it to Maouche but the Frenchman curled his right-foot effort from 20 yards just over the bar.
Bass was called into action again on 33 minutes when Norman Wabo weaved his way into the box.
He angled a shot towards the far right-hand corner but a fully-stretching Bass produced a superb save to flick the effort round the post.
Oxlade-Chamberlain looked a natural at right-back and intercepted a ball laid on a plate to Renei Batlowka from Nico Cotton after the restart.
The 19-year-old, currently out on loan at Poole Town, also impressed going forward.
He produced two sizzling crosses after charges down the right flank, but both balls went begging and were turned away for corners.
Substitute Harry Phillips gave the visitors a much-needed spark.
On 75 minutes, his deflected cross from an acute angle spiralled over Bass but hit the woodwork.
Pompey defended valiantly in the closing stages and, in stoppage time, should have doubled their lead.
The hosts hit Southend on the counter attack and Chaplin was one-on-one with goalkeeper Ted Smith, but Drew Yearwood hacked the striker down.
However, the Worthing-born talent crashed his spot kick off the bar.
There was one last chance for Southend as they searched for an equaliser when Wabo attempted an acrobatic over-head kick, but it sailed over.
The win meant the Blues advance into the group stage of the competition they reached the semi-finals of last season.
They go into Group F along with Everton, Leicester City and Barnsley.
POMPEY: Bass; Oxlade-Chamberlain, Thompson (Dandy 60 mins), Casey, Hancott; Widdrington, Maouche; Smith, Bedford (Lethbridge 82 mins), Mayes (Chandler 88 mins); Chaplin