Hisoka is always looking to fight a strong foe. That is his life's goal, to fight the strongest around. It's what gives him the will to live. In his eyes, Chrollo is one of the strongest Nen users in Hunter x Hunter. Without his abilities, Hisoka won't even look at Chrollo. This is shown when Kurapika uses Judgement Chain on Chrollo. Chrollo cannot use Nen, and Hisoka leaves without fighting him since he says he is not worth dealing with in that state.