Gillingham would have been expecting an improved second half performance from Pompey at Fratton Park.
But the visitors were left stunned by a display of relentless attacking quality in the EFL Trophy group stage contest.
It had been level at the interval, with Tino Anjorin’s overhead opener cancelled out by Jonny Williams.
Ben Stevenson, Kusini Yengi, Abu Kamara and Ryley Towler all struck following the restart to seal a 5-1 victory, though, and left John Mousinho delighted.
The Blues boss said: “It was a pretty emphatic second half after what was a poor first one – we just weren’t at the races.
“But for everything that we got wrong before the break, we then got right after it. Credit to the players for that.
“To put the game to bed and make it as comfortable as they did is something that I thought was really pleasing.
"For everything that we got wrong before the break, we then got right after it. Credit to the players for that."
“It was a joy to watch. I didn’t have any frustration with the way we pressed and there was just one chance they had at the end when we switched off defensively.
“The only criticism is that we didn’t put the ball in the net a couple more times, but that’s being really picky.”
Anjorin made way for Joe Morrell at the break, while academy duo Koby Mottoh and Mitch Aston were brought on for the closing stages.
Mousinho said: “It was always a pre-planned substitution for Tino because we didn’t want to risk him for any longer than that.
“He scored our first goal and showed some real bursts of athleticism and that quality on the ball we know he’s got.
“Joe came on and played really well, but the other 10 guys on the pitch also really stepped their game up.
“Koby’s played a few games for us now and it was nice to give him another run-out, while Mitch deserved his debut because of the feedback we’ve received from the academy.”