Who could beat Nadal on clay if they got to serve every game ?

Don't know if you can be definitive about it, but here is a list of the top ten service game winners so far 2012 (all surfaces).

1. Raonic
2. Federer
3. Isner
4. Del Potro
5. Nadal
6. Karlovic
7. Berdych
8. Almagro
9. Lopez
10. Djokovic

if djockovic served well hes very capable of beating nadal even on clay. raionic would have a chance but think nadal would win a few points on tb even break him 1 r 2 times with his return game

Pete Sampras

Not worthwile looking anywhere else. Ask Roger

I think the list would be a bit longer than just a few names. By the way who would be the lowest ranked player wh could have a real chance? I dare to say Olivetti could have an outside chance.

Isner they says 

What you're asking is; who would Nadal break on serve less than 40%-42% of the time - I dare say the list should be longer than the 5 the OP mentioned.

berdych serves well and can get into rallys. but hard to look past djocko or fed and even they will struggle.lets face it raionic is not breaking nadal