There are a number of ways to be bias:
*Double standard. The most common method that's use, and gets exposed often.
*Exaggeration. Making mountain out of a molehill when certain facts suit their agenda, but vice-versa when it works against them.
*Double-edged sword. I.e. If a player is too dominant, the field is weak. If a player isn't that dominant, it's because the supreme strength/depth of the field.
*Hide negative facts. Only employ by the old-timers. They know the younger fans doesn't know about history of classic tennis, so they take the advantage.