The Ivorian tapped in after Fernando Torres' hard-working assist to top off an unspectacular but determined away victory in Portugal, setting the Blues up well for the second leg
The first half started very slowly but things heated up after the break, Jardel coming close with a free header before Fernando Torres' impressive assist from the right wing opened up space for Kalou to tap home untroubled with 15 minutes to go.
The home side lined up with reported Manchester United targets Nicolas Gaitan and Javi Garcia included in midfield alongside experienced Argentine Pablo Aimar, with Oscar Cardozo leading the line.
Roberto Di Matteo, however, opted to leave veterans Didier Drogba, Frank Lampard and Michael Essien on the bench, with Torres starting up front and David Luiz at centre-back in place of Gary Cahill, while Paulo Ferreira made a shock start, his first of 2012.
The opening stages were cautious and scrappy, the game not seeing any sign of life until, in the 19th minute, Cardozo moved ahead of John Terry to take control of an overhead pass from Bruno Cesar, the tall Paraguayan attempting to volley it in but unable to find sufficient space and shooting wide.
The game became more open, with Torres notably shooting over from distance, but Cardozo remained the main target of Benfica’s attacks, first heading high from an awkward position before forcing Luiz to execute a difficult but stylish juggle of an attempted cross before he could hit it out of play.
Torres was looking lively for the Blues and, on 38 minutes, he demonstrated some fine hold-up play outside the box before taking an ambitious shot, though knocking it high and wide.
Oft-booed ex-Porto man Raul Meireles nearly capitalised with another long-range effort after Kalou teed him up. The Ivorian advanced down the left before pulling it back into the Portuguese’s path, and the midfielder drew a fine low stop from Artur with his driven effort - but half-time arrived with neither side finding a breakthrough.
The second period began with howls for handball. Benfica were forward in numbers for a long throw and Cardozo picked the ball up just outside the area. The Paraguayan unleashed a rasping left-footed shot bound for goal but Luiz reacted on the line to block, to the fury of the home fans who felt that he had handled.
Chelsea, though, should arguably have taken the lead a moment later. Torres continued his fine build-up play, this time cutting in from the left to dink a cross in for Kalou, who had stolen a crucial inch from his marker in the area, but the Ivorian’s header ballooned ineffectively for naught.
After an optimistic penalty shout from Benfica when Maxi Pereira’s shot cannoned back off Terry’s arm following an admirable Ashley Cole block, the Blues’ clearest chance arrived when Jardel failed to anticipate a long ball forward and allowed Juan Mata to run in behind him. The Spaniard circled Artur but was unable to apply the necessary sidefooted finish, hitting the post fruitlessly as the angle closed too far.
Axel Witsel gave Cech a scare on 65 minutes when, having scrapped past John Obi Mikel, the Belgian’s shot deflected heavily off Terry. The Chelsea keeper was nowhere but watched it land on top of the net with relief.
Benfica kept coming, crossing once from a corner and being repulsed before Gaitan delivered a second ball, this time finding Jardel unmarked at the post. His header was poor, however, and Cech’s good reactions were enough to keep it out.
Chelsea, though, found the vital away goal on 75 minutes. Ramires fought hard on the right of midfield, supplying Torres further forward on the wing. The Spaniard did most of the work, closing in with purpose before feeding Kalou in the middle. With Artur taken out of the picture, the forward needed only to poke home simply to take the lead.
The home side pushed desperately to even things up but it allowed the Blues to break, substitute Daniel Sturridge stretching play before passing to Mata on the left. The Spaniard looked to lob Artur but lacked accuracy and the chance was lost.
Benfica put hearts in mouths when Cole was forced to stick his leg out wildly to block a cross in stoppage time, the ball diverting just wide of his own goal, but they lacked punch and Chelsea were able to see the game through to assume control of the tie.