You cannot just use the demonstrative pronoun αυτός/ή/ό without the definite article. It's just a language particularity... of the English language that it doesn't use it too! :P
Edit: To the contributors, if one of the two final ν's is needed, it's the other one: "Αυτήν τη φορά". I have to admit it looks a bit weird in the pronoun but that's how it should be; the second is optional, depending on when someone was at school (currently it's taught to be dropped).