It's tough' It's not what you do, but also how you do it. Tennis being a one on one sport your advessaries have a lot to do with it. But #'s alone put Larry Holmes ahead of Muhammed Ali. At their respective peaks Ray Mercer would be ahead of Ali.
Like the OP said Federer's French Open finals, give him the nod if they are tied at 14. OK but consider this, their 4 best runs in Paris....
-Sampras (3 QF, 1 SF), Federer (1 SF, 3 F)
...on the surface Roger's looks better. But look at who the played when the y reached the QF, Sampras (Hanescu, Ancic, Robredo and Gonzalez), while Federer (Hanescu, Ancic, Robredo and Gonzalez).
Now if we were to interchange hem it's plausible that Roger may not see a French Open F, while I'm sure Sampras would rather go up against Hanescu, Ancic, Robredo and Gonzalez rather than Hanescu, Ancic, Robredo and Gonzalez. Now I would give Roger the nod over Pete on clay, but if interchanged, Pete could have a couple of French Open finals, while Roger would have his hands full. Based on that would Pete get a nod based on the fact he made the French Open F???
Also why is clay made the decided factor. Personally I would take Pete on grass and carpet. Roger on clay, and if you call hardcourts even, you can do the math there. So it's not that easy.
But I do love the fact that even at 14 people would find any reson they could not to give Sampras the thrown, but now the Fed is approaching, it has now become the stamp of approval for the GOAT title.