View the photos from the Blues' dramatic Stadium MK triumph
Pompey scored two late goals to battle from behind and secure a 2-1 victory at MK Dons.
Ike Ugbo fired the hosts ahead midway through the first half and Luke McGee made several saves to keep the Blues in the contest.
Brett Pitman was denied by Lee Nicholls at the other end, while Oli Hawkins struck the woodwork as an equaliser appeared elusive.
But Matt Clarke headed home with time running out and substitute Conor Chaplin then swept in a stoppage-time winner.