[Formendacil]

Sauron would not necessarily have expected a one-on-one fight. The Eagles, after all, were the messengers of Manwë, and their appearance would mean a certain blessing from the Valar- a blessing that a one-on-one fight USING the Ring would never have garnered from the Valar.

Furthermore, anyone with strength half-ways capable of challenging Sauron with the Ring would have spent time using it, becoming stronger. They wouldn't hop on an eagle and head straight to Mordor! Especially if they were wise and powerful enough in the first place to convince an Eagle to take them there...

Anyway, that's my opinion. You don't have to accept it...