It is now accepted! Probably took Duo a couple of weeks to accept your suggestions... but they did it :)
You can also use "favorito" which is a lot easier to remember and logical!!
Did not accept "preferred." Just as there's a differnce, in English, between preferred and favorite, there is surely the same difference in Spanish. Something can be "preferred" (preferido) between two or more sports and not be someone's "favorite" (favorido) sport.
Because the sentence is talking about tennis in general. In Spanish when discussing a generalized topic you use the article.
2019-04-12 A quick Google search turns it up in the phrase Noticias de Tenis (on ESPN en español), and el circuito…de tenis. So it looks like the article is not needed when a noun is being used as a modifier for another noun.