Source : Webster's Revised Unabridged Dictionary (1913)

Fanatical \Fa*nat"ic*al\, a.
   Characteristic of, or relating to, fanaticism; fanatic.
   -{Fa*nat"ic*al*ly}, adv. -- {Fa*nat"ic*al*ness}, n.

Source : WordNet®

fanatically
     adv : in a passionately fanatic manner; "he followed the teachings
           of his guru fanatically"