The rivals clash at Stamford Bridge on Wednesday with the Blues in the driving seat after leaving the Estadio da Luz with a 1-0 victory last week. A semi-final against Barcelona or Inter Milan awaits on April 18 should they dispose of Benfica, but Kalou is wary of complacency.
"I think our worst enemy can be ourselves, thinking that we have done the most difficult part," he said.
"We have to keep going, working hard. We have to prepare and try to score early and secure the victory at home.
"It's not finished yet. Benfica are a good team and we have to respect them.
"We have to play 100% in the second leg if we want to go through to the semi-final.
"We know that scoring in the away game is always important, and we haven't conceded.
"It's still going to be a difficult game because Benfica are a great team, as they showed against Manchester United before, so we have to respect them."
Source: PA
Source: PA