Blues captain John Terry was sent off in the first half as Barca went two up before the visitors managed to snatch two counter-attack goals at the end of either half to progress.
"It's an incredible achievement by this group of players," Di Matteo told Sky Sports News. "We showed again what kind of character we have.
"We didn't expect to play with 10 men, we knew it was going to be tough. Barca are a great team but we showed what we're made of."
Speaking of Terry, Di Matteo added: "He's a fantastic leader of this group. Everyone can make a mistake in life.
"I'm not angry with him. We're all human beings."
Chelsea will be without Terry as well as Branislav Ivanovic, Ramires and Raul Meireles after the trio picked up their second bookings at the Camp Nou.
Gary Cahill also came off injured to leave the West London club with a slight injury dilemma.
Di Matteo, however, wanted to enjoy the moment and worry about selection problems at another time.
"Let's enjoy it for a moment. We achieved something incredible tonight."
The Italian's men will take on either Bayern Munich or Real Madrid on May 19 at the Allianz Arena in Munich.