Litotes, you lost the argument.
Saying "Djokovic will win at least 10 slams" is a prediction about a future tennis event.
If Sampras does not make accurate predictions about future tennis events, then there is no reason to believe what he says is correct here. (Excluding any existing reasons for believing this, of course).
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.