The west Londoners never looked in any real danger of slipping up at Stamford Bridge as they netted four times in the second half of the third-round tie.
Juan Mata directed home a Florent Malouda cross to give Andre Villas-Boas' side the lead three minutes into the second half before the npower Championship club pushed for a leveller.
However, Chelsea secured progression to the fourth round as Ramires grabbed a late brace and Frank Lampard scored deep into stoppage time.
"I thought it was a very good all-round performance," Chelsea assistant manager Di Matteo said.
"Portsmouth made it very hard, especially in the first 45 minutes, and it is always difficult to break an opponent down and find a space with so many players.
"It was just a matter of scoring the first goal and we were always confident we were going to be able to do that.
"In the second half we got further forward and secured the win.
"I think it was a matter of finding the spaces and keeping possession.
"We knew the chances would come and that is exactly what happened in the second half."
Di Matteo said their was never any danger of Chelsea playing a weakened side.
He added: "I don't think we ever said we wouldn't play a strong team so it wasn't really a surprise.
"It is a competition we would like to win and today [Sunday] we have gone a step further towards the target."
Ramires was one of the west Londoners' standout performers at Stamford Bridge.
The Brazil international came close on several occasions against Pompey before tapping home from close range in the 85th minute and then clipping the ball over goalkeeper Stephen Henderson two minutes later.
"He helps things," Di Matteo said of Ramires.
"I don't think he is the only one as we have a few goalscoring midfield players.
"I very much enjoyed the second goal he scored today [Sunday].
"I think the scoreline was a bit harsh on Portsmouth, especially the way they were organised and defended.
"We are very pleased with it and we deserve a few goals as well."
There had been speculation Bolton defender Gary Cahill may have completed his expected move to Stamford Bridge in time for the match, but the deal has still not yet been completed.
Di Matteo claimed he did not know how the deal was progressing and was even less forthcoming when asked about reports Arsenal are interested in Salomon Kalou.
"I don't know, I am not in the loop [with the Cahill deal]," the Italian said.
"Talks are still ongoing but I don't know when it is going to happen."
On a potential move to north London for Kalou, he added: "I know he is at the African Nations Cup with the Ivory Coast mini camp in Abu Dhabi. That is what I know."
Victory sets up a mouth-watering trip to west London rivals QPR, assuming Rangers can defeat MK Dons in their third-round replay.
For Portsmouth, though, attention returns to the npower Championship after a slightly cruel defeat for Michael Appleton's side.
"Conceding after two or three minutes of the second half does not help matters, especially after a good first-half performance," the Pompey manager said.
"I thought we got back into the game and you could sense their was a bit of nervousness before the end when they got their second goal.
"I think [the performance] was a credit to the boys but that is why Chelsea are where they are and we're a Championship club.
"The longer the game goes on, fatigue comes in, a lack of concentration comes in at times and obviously they have got players that can be clinical."