Groth is serving his way to the title in Memphis this week
I've obviously voted for the Kokk since I think that when he does get it together he will do a Kyrgios-like deep run and jump up fast.
But Groth would certainly be my next pick given his continuing improvement, and he could well get there before the Kokk manages to pull it together, and yes even this one is possible (Karlovic could take out Nishikori for him!).
This tournie though, he has to get past Mr Tomic first, which could be an interesting test of Groth's learning rate and Tomic's determination to stay on track...