I think Nadal - Djokovic semi-final would be an incredible game.

Federer's best chance of finally winning in Paris would be for Djokovic to beat Nadal.
I reckon Federer is psyched out by the combination of Nadal and clay, and playing Djokovic in the Final would enable him to play his best.

But Nadal is just a monster player on the surface, he lifts his game when playing Federer.