Conor Chaplin scored the only goal as a youthful Pompey Reserves side beat Southend to reach the group stage of the Premier League Cup.
The striker also missed a late penalty, while Nathan Thompson played almost an hour in his comeback from a foot injury.
Barnsley, Everton and Leicester now lie in wait for the Blues, who reached the semi-finals of this under-23 competition last season.
Alex Bass made a simple save to deny Michael Klass in the opening stages, but it was the hosts celebrating an early goal.
Theo Widdrington’s corner led to a scramble in the Southend box and CHAPLIN – showing typical predatory instincts – arrived to poke home.
Trialist Mohamed Maouche almost doubled the advantage soon after, but his 25-yard effort narrowly cleared the crossbar.
The Shrimpers were causing few problems in attack, while the Blues were working hard to break their opponents down at the other end.
But they received a warning when Norman Wabo burst through and angled in a shot that Bass just about got his fingertips to.
And the teenage keeper was called into action again moments later, this time smothering the ball at the feet of Wabo.
It was Pompey who finished the half on top, however, and a delivery from Matt Mayes flew tantalisingly over the head of a lurking Chaplin.
Half Time: Pompey 1 Southend 0
The visitors almost capitalised on some defensive uncertainty early in the second half, but Bass kept out Nico Cotton’s effort.
Christian Oxlade-Chamberlain soon created an opening at the other end, bursting up the pitch and sending in a cross that somehow evaded two of his team-mates before being forced behind.
And the academy graduate then did even better at the back to prevent Renei Batlowka collecting Cotton’s incisive pass.
Southend were swarming all over their hosts by this point and Pompey just about held on when the ball ricocheted around their box.
There was another scare on 75 minutes, as Harry Phillips’ deflected cross beat Bass and came back off the post.
But the Blues then almost added a second on the break, with Elvis Bwomono hacking Chaplin’s effort off the line.
And Chaplin really should have wrapped up the victory in stoppage-time after being tripped in the box by Drew Yearwood, only to send his spot-kick crashing against the crossbar.
Southend tried to make him pay for his profligacy by sending everyone up for a last-gasp free-kick, but Wabo’s acrobatic effort flew over.
Pompey: Bass; Oxlade-Chamberlain, Thompson (c) (Dandy 58), Casey, Hancott; Smith, Widdrington, Bedford (Lethbridge 78), Maouche; Chaplin, Mayes (Chandler 87)
Subs not used: Collins, Brooks
Southend: Smith; Bwomono, Coutts, Matsuzaka (c), Kyprianou; Bedford (Phillips 59), Yearwood; Batlowka (Marah 80), Klass, Cotton; Wabo
Subs not used: Bishop, Clifford, Clark
Referee: Tristan Greaves