The Blues lead 2-1 heading into the return bout at Stamford Bridge and although they have not lost at home in European competition for 12 matches, Mata is not getting too carried away.
"It's going to be a dangerous game," Mata said.
"Even though we have two goals away, a great result, it's that kind of game that can be very risky.
"If we don't go into the game thinking about winning, thinking about defending well, thinking about trying to win again, it's going to be difficult.
"We still know we have to fight if we're going to be in the final.
"We got a great result in Switzerland, but now we have to play another 90 minutes and I'm sure they're not going to give up.
"They're a team who try until the end. It will be tough, for sure."