Pompey remained firmly in the contest at the interval of their Carabao Cup second round tie at Brighton.
Trailing to Alexis Mac Allister’s header, they had looked bright going forward and created a few chances.
But the Premier League side took control in the second half, with Alireza Jahanbakhsh, Bernardo and Viktor Gyokeres all netting in quick succession.
Kenny Jackett admitted that the Seagulls deserved their spot in round three and was disappointed that his side could not make the most of their attacking play.
The Blues boss said: “We played pretty well going forward and created opportunities, particularly from set-pieces – we got into double figures from corners.
“But we unfortunately couldn’t capitalise on that and Brighton had both the power in both boxes and the football in between them.
"Some of the goals we conceded were certainly avoidable, but we have to keep progressing and working away."
“We accept that they deserved to win the tie, although there were positives in terms of the positions we got into.
“Some of the goals we conceded were certainly avoidable, but we have to keep progressing and working away.
“There is a good attitude in our group of players and that’s something we’ll need during a long season.
“It’s about getting that balance right between being stable defensively and then constructive going forward.
“That’s the same for any game you play, whether it’s the cups or, as we look ahead to Sunday, the league.”
Jack Whatmough appeared to hobble off when he was replaced by Paul Downing in the second half, but Jackett moved to delay any fitness fears.
He said: “He wasn’t injured – it was just a dead leg and meant he was limping slightly. He could have played on and is confident of being able to train tomorrow.”