And not, "widely regonized" is not a proof. Millions of people may believe/think something and still be wrong.
"Proof beyond a reasonable doubt does not mean proof beyond all doubt. It does not mean proof beyond a whimsical or an imaginary doubt, or proof beyond all possibility of innocence. A reasonable doubt does not mean the kind of doubt that could be conjured up by someone who is
seeking for doubts or for an excuse to acquit a defendant. Instead, a reasonable doubt means such doubt as remains in the mind of a reasonable person who is earnestly seeking the truth.
If you can provide a reliable source which says that a person is widely regonized as a tennis expert by tennis experts, that person is beyond a reasonable doubt a tennis expert.
Knowledge about what subject? How to hit a tennis ball? What the results of past tennis matches are? How to report on a match effectively?
An expert on what? I'm sure Sampras is widely regarded as an expert on hitting awkward volleys and making booming first serves. Does that mean he can tell us what the top 10 will be 5 years from now?
Pete Sampras was a top professional player in tennis for a long time, so it's logical that he is very knowledgeable about tennis.