Thank you. I now understand that it is a masculine noun ending in 'a' because it is from the Greek and ends with 'ta'. :)
Actually atleta can be both masculine and feminine : un atleta uomo e un'atleta donna. They are different in the plural : atleti uomini e atlete donne.
That makes sense and in the singular you can tell which gender is intended by the article and any adjectives that agree. Thanks! :)