The reigning champions travel to Stamford Bridge on Wednesday for the first leg of their last-four tie.
Despite Barca being favourites to reach the final, Guardiola understands that his team have a tough task against Roberto Di Matteo's men.
"Chelsea have shown they are capable of anything and have fantastic players in every way," he told The Mirror.
"We will have to play a good tie to reach the final. Chelsea can knock us out."
Barcelona are hoping to become the first team to retain the Champions League trophy since it was rebranded in 1992.