We describe the conception of a new experimental setup to manipulate the internal state of a Rydberg atom inside the Stark manifold using a well-polarized radiofrequency electric field. This setup allowed us to transfer with a nearly unit efficiency the atoms from an optically-accessible low-m state to the high angular momentum circular Rydberg state. We then use the setup to observe the quantum Zeno dynamics in a large Hilbert space. By applying a well-chosen microwave field addressing a specific state, one can restrict the atomic evolution induced by a radiofrequency field to a subspace of the Stark manifold. This confined dynamics is very different from a classical dynamics. The system periodically experiences a phase inversion, transiently being in a « Schrödinger cat state ». We experimentally observed this evolution in the phase space and measured the atomic Wigner function at the phase inversion time. This is the first demonstration of the non-classical nature of the quantum Zeno dynamics in a non-trivial Hilbert space.