There were just 93 seconds on the clock when Louis Dennis opened his account for Pompey.
The summer signing rose to head them into the lead at Southend in a Checkatrade Trophy third round tie.
Gareth Evans added a second goal on the stroke of half-time in what was a comfortable 2-0 win for a largely youthful Blues side.
And Dennis was pleased to play his part in a result that takes Pompey into the last eight of the competition for the first team.
He said: “It’s a lovely feeling to finally get off the mark and, more importantly, to get through to the next round.
“I don’t get many headers, but I felt I got up well and just tried to direct the ball – luckily it hit the back of the net.
"It’s a lovely feeling to finally get off the mark and, more importantly, to get through to the next round."
“Scoring so early gave us a platform to build on and that’s exactly what we did with the second goal coming just before half-time.
“That was important and from then on it was just about staying organised to make sure we held on to the lead.
“We came here with the right attitude and were prepared to dig in at times. If you keep a clean sheet then you give yourself a good chance.”
Dennis has been limited to Checkatrade Trophy appearances since arriving from non-league Bromley back in May.
He said: “It was nice for me to get 90 minutes under the belt and I won’t stop knocking on the door for more games.
“The team’s been doing well lately, but you pick up a few injuries here and there, so always have to be ready.
“Things don’t necessarily fall your way in football and it hasn’t really gone for me yet, although there’s still half of the season left.”