The last five sets between these two on clay have been 6-3, 6-0, 6-1, 6-2 and 6-1, all in favor of Del Potro.
Surely tomorrow won't be this one-sided, but Del Potro must be the favorite. He is the better player overall, has a tremendous match-up advantage (similar style, but his groundstrokes are more powerful and consistent) and he is more comfortable on clay than Marin.
That said, Marin is in good form and on a good day he can upset Del Potro for sure. Nothing is set in stone and Marin is capable of producing awesome tennis.
Cilic is unduly criticized and ignored here and is capable of giving almost anyone a tough match. Odds are on Del Po obviously but could be entertaining.