Learn Spanish (Latin America): El tenis es tan divertido como el fútbol.

When comparing adjectives to say "as [adjective] as", Spanish uses tan + adjective + como. Because divertido is an adjective, we use tan. You would only use tanto (or its forms like tanta) when comparing nouns, such as "as much money as" (tanto dinero como).

Why are there "el"s before the sports, like el tenis and el fútbol?

In English, we just say "Tennis is as fun as soccer" without the word "the". But in Spanish, when you talk about sports, hobbies, or broad concepts as the subject of a sentence, you generally have to include the definite article (el, la, los, or las).

If we compared feminine sports, would tan or como change?

No. The adverb tan and the word como never change their form. However, the adjective between them still has to match the subject. So if you were talking about swimming (la natación), you would say La natación es tan divertida como... where divertida is feminine, but tan and como stay exactly the same.

Why is there an accent mark on fútbol?

The accent mark tells you to put the spoken emphasis on the first syllable (FÚT-bol). Without it, the normal rules of Spanish pronunciation would force the stress onto the last syllable, because the word ends in an 'l'.