The Blues go into the match 1-0 in front from the opening leg in the Czech Republic last week.
Despite the fact that they have an away goal in their favour, Cahill has admitted that Chelsea saw enough in Prague to know that Sparta will provide them with a stern test for a second time.
"You've got to take teams seriously. They're a quality outfit and made it tough for us away from home," Cahill told reporters. "We're playing at Stamford Bridge and are always confident but we won't take it lightly.
"They made it difficult for long periods of that game. In Europe, our home form, we're unbeaten in quite a few games and hopefully it can continue."
Chelsea have not lost any of their last nine European matches at home.