Crawley Town head coach Dermot Drummy admitted Pompey’s two opening goals put them in the driving seat.
The Reds fell 2-0 behind in the first 12 minutes when goals by Curtis Main and Gary Roberts.
The goals ignited the 16,000 crowd to create an initimidating atmosphere.
Portsmouth sealed victory in the 37th minute when Main added a third goal.
Drummy said: “The plan was initially to sit and to try and hit them on the counter-attack but I think sloppy play giving two goals away put them in the driving seat for the game.
“And then the third one just before half-time has killed us.”
Drummy believed his improved in the second half after he made changes which enabled them to keep more possession and manage to avoid conceding more goals in the second 45 minutes.
The Reds boss said: “Josh Payne came in the middle and kept the ball a lot better. Bobson up front gave us a spark and Jason (Banton) on the left with Chris Arthur there gave us more emphasis going forward.”
He added: “But they were probably content to contain us if I’m honest.”
Drummy admitted defender Alex Davey’s mistake which led to Portsmouth’s second goal was costly but the Chelsea loanee would have to learn from his blunder.
He said: “It’s a learning curve, that’s life, you have to deal with it, he’s made an error, you will see a few of those on the telly over the weekend but he has to pick himself up and there were others who weren’t playing the way I want around the back either.
“Some people don’t want the ball at times and they don’t make space for people.
“You’ve got to have that bravery to play, not suicidal into their midfield block, that’s when you should be going in behind.
“In the first half they pressed us and we still didn’t turn them around, we still tried to play poor balls and not good quality balls.
“We’ve come to one of the strong favourites to go up and got well beaten.
He praised their opponents and the 16,000-strong Fratton Park crowd.
“Portsmouth’s movement, their pressing and their fans, it’s some place to come, it’s some atmosphere here.”
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