The England defender praised the Blues' discipline following their 1-0 first leg victory against Pep Guardiola's team but is not underestimating the task ahead at Camp Nou.
Didier Drogba scored in first-half added time and the Blues defense was able to withstand the pressure applied by Pep Guardiola's team to secure a narrow advantage to take to Spain.
However, Cahill is under no illusions as to the difficulty of the task waiting for the club when it meets Barcelona again on Tuesday.
"It was a fantastic result but it's only halftime," Cahill told The Daily Mail. "I thought as a team tonight we were well organized. Everyone was on the same page.
"The gaffer gave us a game plan before the game and we stuck to it. It worked really well, but it's halftime and it's going to be really hard over there but it's a good start for us."
Cahill identified Chelsea's focus and willingness to carry out Roberto Di Matteo's game plan as the main reasons for the victory at Stamford Bridge.
"Defensively, I thought as a team everyone stood up to their task and the reason we won was because of the concentration levels," Cahill explained. "The shape was a bit more defensive than we are used to but you have to play like that considering the amount of possession that they have.
"There's always an element of luck you have to ride against them but we stuck to our task."
The center back recovered well to dispossess Lionel Messi in a dangerous position in the second half and expressed his delight at the reaction of the Chelsea supporters, who chanted his name.
"I could hear that being sung and it was obviously tremendous and a proud moment for me," Cahill enthused. "You can't get the ball off [Messi] can you? Even if you have four or five surrounding him, but I managed to get the tackle in."
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