Originally Posted by TheCheese
I think you might be having a hard time getting a feel for where his racket is in relation to his forearm from this camera angle. Check out the side view.
Pronation isn't going to make his racket tip point at the camera. All it's doing is closing his racketface towards the ground.
Check out the pictures that toly posted about forearm and wrist movements. This is my understanding of the definitions.
In the picture of Fed that you posted the racquet is making very roughly a 90° angle with the forearm, true? That means that the racquet, more specifically the line from the handle to the tip, is in a plane that is normal to the forearm axis. Let's define the positive direction of the forearm axis from the hand to the elbow. Let's define the positive direction of the racquet axis as from the handle to the tip. Pronation would therefore be a positive rotation (right hand rule) of the racquet axis about the forearm axis.
Because racquet axis and the forearm axis are normal, neither pronation or supination will cause a rotation about racquet axis, which would be required to open or close the racquet face.
In the picture you provided a human should be able to pronate their forearm about 90° from the position shown. The racquet would be point more toward the camera, though in this case not at the camera. In your picture Fed's racquet is not quite in the same position as in the other Fed picture. His upper arm as rotated so that the racquet is pointing a bit more backwards.
Please remember that I am making some approximations on some of these positions. His racquet and forearm are not exactly normal (90°), but they are certainly not in-line. His forearm isn't pointed straight down but rather at an angle to the ground, so the plane normal to the axis of the forearm is not parallel to the ground.
Sorry for all of the engineering/math speak, but I don't know how else to unambiguously communicate my meaning.