Portsmouth's Miles Storey starred on his full debut for the club, firing them into the Capital One Cup second round as his side sunk Peterborough 1-0 at Fratton Park.
Pompey outclassed their Sky Bet League One rivals in the first half with Storey - forced off with an injury after 68 minutes - terrorising Posh on the right.
Storey struck after 12 minutes, finishing coolly across goalkeeper Ben Alnwick after Dan Butler and Craig Westcarr had combined to find him in acres of space on the right of the penalty area.
Peterborough striker Kyle Vassell should have pulled his side level two minutes after half-time when he weaved his way through the Portsmouth defence only to lash his attempt over the crossbar.
Posh thought they had equalised after 55 minutes, but Conor Washington's finish was ruled out for offside.
Pompey's Westcarr also had a strike chalked off after 71 minutes when he was adjudged to have handled as he dispossessed Michael Bostwick.
Peterborough piled on the pressure in the closing stages, but Pompey sweated their way through six minutes of injury time to hold on.