Pompey collected a bonus point for their penalty shootout win against Oxford United to ensure they finished top of Southern Group B of the EFL Trophy.
The Blues were held to a 2-2 draw at the Kassam Stadium, with Brad Lethbridge netting his first senior goal and Anton Walkes also on target for the visitors.
But the Blues triumphed 5-4 on spot-kicks to finish on eight points after three matches and guarantee a home tie in the second round of the competition.
Kenny Jackett made 11 changes from Saturday’s League One victory at Doncaster.
Among them was Harvey Rew making his Pompey debut aged 17 years and 14 days, while Paul Downing returned to wear the captain’s armband.
The Blues perhaps should have taken the lead in the 11th minute when Ryan Williams pulled a ball back for Lethbridge, but he shot wide.
Four minutes later, visiting keeper Bass was forced into action. Matty Taylor skipped past Walkes before having his effort kept out at the near post.
The U’s started to put Pompey under a sustained spell of pressure and got their reward on 21 minutes.
Shannon Baptiste slipped in Taylor and he made no mistake this time when he fired home from close range.
And just three minutes after the restart, the Oxford striker almost doubled his tally but was foiled by Bass before chipping the rebound wide.
But against the run of play, Jackett's troops equalised in the 33rd minute.
Josh Flint's deflected shot fell to Lethbridge at the far post and his instinctive finish squirmed its way through home stopper Simon Eastwood.
On the stoke of half-time, Bass was forced into action with a top-drawer save.
Josh Ruffles’ cross found Rob Hall arriving late at the back post and the U’s right-back must have thought he'd scored only for his effort to be tipped over the bar by the Fratton academy graduate.
Pompey made a double substitution at half-time, bringing on Gareth Evans and Ben Close for Rew and Flint.
They started brightly, with Cannon and Leon Maloney both having early attempts.
There was a heart-in-the-mouth moment for Pompey on 62 minutes when as Ruffles' low cross found Taylor.
Again, though, Bass thwarted the on-loan Bristol City man before Sykes fired wide from the rebound with the goal at his mercy, although his blushes were sparred by the offside flag.
And the 21-year-old, who is deputising for Craig MacGillivray while on Scotland duty, refused to be breached when preventing Sykes in a one-v-one situation.
Despite being forced into a rearguard action, Evans had a gilt-edged chance for the away side in the 71st minute when he was set free by Maloney, but saw his shot saved by Eastwood.
Oxford began to pile on the pressure – but couldn't find a away beyond Bass when he saved Brannagan’s powerful shot with 15 minutes remaining.
The Blues displayed their prowess on the counter and Walkes must have thought he'd given his side the victory on 85 minutes.
Cannon's lofted pass picked out the run of the right-back and he poked beyond the onrushing Eastwood.
But Karl Robinson's men would hit back in stoppage-time.
Rob Dickie had been a handful throughout from set-pieces and on 92 minutes his finish on the spin ensured it finished 2-2 – meaning extra point was up for grabs from the penalty shootout.
Ben Close missed for Pompey but Bass’ excellent save to keep out Ruffles’ effort took it to sudden death.
Both Walkes and Dan Agyei were unsuccessful, but Brandon Haunstrup kept his cool before Long shot wide to give the Blues a 5-4 win.