Novak Djokovic (SRB)[3] v. Rafael Nadal (ESP)[2]
Nadal of Spain has won the French Open each of the three years he has played it. I could list two or three paragraphs of Nadal's claycourt accomplishments. But I'll just say he is the best claycourter by far and has been without any claycourt rival of any sort for a number of years.

On another surface I might pick Djokovic of Serbia to win. Djokovic improved his game dramatically earlier this year to take the Australian Open title. On this surface however the combination of Nadal's lefty serve to the opponent's backhand and Nadal's penetrating groundstrokes are almost impossible to beat.

Nadal time after time breaks down his opponent's backhand before the match is over. Djokovic trails Nadal head to head 3-7 and has never beaten Nadal on clay.
Winner: Nadal

Roger Federer (SUI)[1] v. Gael Monfils (FRA)
Number 59th ranked Gael Monfils' improbable French Open run appears to be fast approaching an end. World number one Federer holds nearly every advantage imaginable except a few possible key factors. Federer leads Monfils 3-0 head to head including two victories this spring. Federer will also have the experience factor and most likely a very consistent game on his side.

But Federer's worst surface is clay and Monfils will have a tremendous crowd support. Monfils also has a height advantage versus Federer as he at 6' 4” is three inches taller. If Monfils can serve big and play a good first strike baseline game, he could have a chance to upset Federer who would desperately prefer a faster playing surface.

Anything other than a Nadal versus Federer final would be a huge upset but I think that is what is going to happen.
Winner: Monfils