OP included the answer in the question. except Djokovic...
Clay: Horacio Zeballos
or anybody who has nothing to lose and can absorb heavy topspin and hit deep and hard for 2 hours. Juan Martin Del Potro, Tomas Berdych, Lukas Rosol. At RG: best of 5, nobody i'm afraid.
Grass: many top 20 guys in fact, if they believe in their chances. coming to the net is key to victory for most. great serve helps too. best of 3: Tommy Haas
, Tomas Berdych, Grigor Dimitrov, Robin Haase, Marcos Baghdatis. Wimbly: Jo-Wilfried Tsonga
, Marin Cilic, John Isner.
HC: ballbashers, servebots and tacticians. best of 3: Milos Raonic
, Sam Querrey, Kei Nishikori, Gilles Simon. USO: Juan Martin Del Potro
, Andy Murray, Jo-Wilfried Tsonga, Marin Cilic, John Isner, Ernest Gulbis.
Indoor: anybody except Richard Gasquet, Nicolas Almagro, David Ferrer and guys he knows too well to lose to.