Two goals from Fernando Torres and one each from Raul Meireles, Branislav Ivanovic and substitute Salomon Kalou put Chelsea in command of Group E and they will almost certainly take their place in the knockout rounds with another win over Genk in the return leg on Nov. 1.
Meireles fired Chelsea in front after eight minutes and they doubled their lead when Torres, currently suspended from domestic matches, scored three minutes later.
The Spaniard headed in the third after 27 minutes and Ivanovic headed in a rare goal three minutes before the break to make it 4-0.
Kalou made it 5-0 when he fired in from point-blank range just two minutes after replacing Frank Lampard to give Chelsea their biggest Champions League win at the Bridge and equal their biggest victory in the competition, 5-0 at Galatsaray in 1999.
Genk, missing three injured defenders and their influential captain David Hubert offered no threat to the Chelsea defence and their most memorable contribution to the night was their garish crimson strip