Isak headed in Adnan Januzaj's cross shortly after the hour mark at the Ramon Carranza to secure a sixth consecutive league victory for La Real.
An excellent start to the season shows no sign of abating, with Imanol Alguacil's side now three points clear of second-placed Atletico Madrid, albeit having played two games more.
It has scored an impressive 21 goals from their opening 10 matches, the highest tally in the league, and conceded only four.
After travelling to AZ Alkmaar in the Europa League on Friday (AEDT), Real Sociedad play at home to Villarreal, which sits third and are the league's other surprise high-flier.
Defeat means Cadiz sits sixth, just above Sevilla after a strong start for the newly-promoted side. Getafe was earlier held to a goalless draw by Eibar.