Litotes, you lost the argument.
The comparison here is between Sampras and the anonymous poster (Roy Emerson). I know that Sampras is at least some kind of tennis expert while I do not know who is "Roy Emerson". Therefore, there is the reason to have more trust in Sampras than the anonymous poster.
No, the comparison is between believing a prediction to be 'true' or 'very likely' and reserving judgement. Sampras may have more tennis knowledge than "Roy Emerson", and if forced to choose between them, it may be rational to believe Sampras.
But the point is you are not forced to choose between them. If Sampras has not established that he can make accurate tennis predictions, then the most rational thing to do would be to not believe him either. You simply hold up your hands in the air and say "I don't know".