Federer has played Nadal 10 times on Clay and only won once. And everyone of those matches except one have been in the final of a tournament. But if he had been more like Sampras and lost many times early in French open and then never got to face Nadal, then his record against his main rival would be something like 5 to Federer, 4 to Nadal. If Sampras had been better at Clay but not the best, like #2, and consistently lost to a dominant clay courter in finals, then his record would be more tarnished. Ironically Federer being so good an all rounder actually hurts him.