Juan Mata, Ramires and Frank Lampard score for the London club.
Chelsea find the spaces en route to big win over Portsmouth
The Londoners never looked in any real danger at Stamford Bridge as they netted four times in the second half.
Juan Mata directed home a Florent Malouda cross to give Chelsea the lead three minutes into the second half before the 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," 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.
"I don't think we ever said we wouldn't play a strong team so it wasn't really a surprise," he said.
"It is a competition we would like to win and today we have gone a step further towards the target."
Ramires was one of the 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 Stephen Henderson, the goalkeeper, 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 goal-scoring midfield players.
"I very much enjoyed the second goal he scored today. I think the scoreline was a bit harsh on Portsmouth, especially the way they were organised and defended."
There had been speculation Gary Cahill, the Bolton Wanderers defender, 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. "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."
Michael Appleton, the Portsmouth manager, said: "Conceding after two or three minutes of the second half does not help matters, especially after a good first-half performance.
"I think [the performance] was a credit to the boys but that is why Chelsea are where they are [in the Premier League] and we're a Championship club.