actually, note carefully the "sometimes" at the beginning of the sentence.
the connotation here is that he is not always beaten for this reason. "sometimes" other things caused his defeat. you know- like mentally weakened, injured, mono, poor light etc.
But that's perfectly true! Sometimes a player does lose because he played badly. I don't see how you could really argue otherwise.
Federer's point is that he can't always be expected to have a higher level than his opponent and just because his opponent won doesn't mean there was anything in particular wrong with his game - as often defeat comes because his opponent simply executed better on the day. There can
only be one winner after, all.