Didier Drogba scored with Chelsea's only shot on target in first-half stoppage time to give them a 1-0 advantage to take to the Nou Camp for the semi-final second leg, a lead earned despite Barcelona enjoying 72% of possession.
"I am very pleased with this result but it is only half-time," added the Italian. "Nothing has been decided yet and we need to play another game like this one if we are going to get through.
"You know what to expect when you play in the Nou Camp. It is going to be very difficult and we still have to believe we might get a goal out there.
"It is a tough game for any opposition that goes to Barcelona and we will have to give our best again. We are delighted with the result and performance of the team.
"They had a lot of attempts on target, as they always do, which means you need to be clinical when you get your chance, which we were when Didier scored."
Di Matteo also believes Wednesday's result answers the Blues' critics, who have claimed some of their more experienced players' influence was fading.
"In the past, a lot of people have had the opinion that our boys were over the line and too old to play at this level," he said. "It was not just Didier, but a lot of players in our squad, that they said couldn't play two games in such a short period of time.
"They gave their answer on the pitch tonight."
Chelsea are up for it - Drogba
Chelsea had just 28% of possession, but midfielder Frank Lampard said: "There is no shame with looking at the possession percentage after the game.
"Many a great team have been beaten comfortably by Barcelona. If you try and open up against them they have so many tools to hurt you with.
"We wanted to try and counter attack them and hurt them and we had one chance to do that and did it well.
"It is half-time, it's a cliché but true. We did have some luck tonight and are by no means getting carried away with ourselves.
"It would mean everything to reach the final. Two months ago against Napoli we thought we were going out so the opportunity to reach the final is massive."
Barcelona, who created 24 chances without scoring, now face a tough task to turn the result around to go through and face either Real Madrid or Bayern Munich in next month's final in Munich.
Guardiola was asked if he felt, in football terms, that Barcelona had been "mugged" after placing Chelsea under so much possession, only to come away empty-handed.
"If football is about counting possession we would win every game," he said. "The game is about putting the football in the goal.
"It is the most difficult thing in the game. The challenge now is to create 24 chances again but maybe we will need to show more adventure and risk a bit more."
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