Sampras will not hold 60% of his service games against 2008 Prime Nadal. This is the problem with your stats, this isn't the weak field that Sampras had to face, someone up there using Courier as the greatest opponent Sampras faced on clay, we are talking Nadal. You must adjust your figures with factual data, let me give you some.
Alamagro and Verdasco are similar clay court players to Sampras going by their records, in 2008 Nadal beat Verdasco 6-1 6-0 6-2, and then Nadal beat Alamagro 6-1 6-1 6-1.
The two Spanish players above are far far far more accomplished in the baseline game, Sampras the only thing he has in his favour is serve, but as we have discussed the surface nullifies that, and the proof is he was beaten by journey men in his French Open career.
Explain why Nadal beat these two players 6-1 average throughout, and he would do better against Sampras because of his weaker baseline play.