You can say a soccer fan or soccer fanatic. Using "fan" is rather more common. This is better than "a fanatic of soccer".
"Zealot" (notice the spelling) suggests someone who has very strong opinions about something and tries to make other people have them too. Cambridge
The word zealot has rather negative connotations. A soccer fan will spend lots of money to buy a replica kit for their team. But a zealot will organise "anti-baseball demonstrations". We might talk about a religious zealot, in a negative way.
The expression you want is "fan" or perhaps "fanatic".