You don't. But what evidence that people have on this board that their opinion matters more than his? People spend enormous amounts of time on this board trying to prove their own opinion is greater than someone else's. If you want to state your opinion, then that's fine, I can respect that, the same level of respect I'll grant Pat Cash's. However, if you want to push you are right over someone else, the contribute some evidence or knowledge, that is beyond the evidence of the person you are trying to over proving.
For instance, if you've sustained the same type of knee injury that Nadal had, I'd be more inclined to believe how you felt Nadal felt, over someone who've only had a Migraine. If Nadal had a migraine, then I'd believe that person, if you've never had a migraine.