  We investigate the dynamics of a strongly-driven, microwave-dressed,
donor-bound electron spin qubit in silicon. A resonant oscillating magnetic
field $B_1$ is used to dress the electron spin and create a new quantum system
with a level splitting proportional to $B_1$. The dressed two-level system can
then be driven by modulating the detuning $\Delta\nu$ between the microwave
source frequency $\nu_{\rm MW}$ and the electron spin transition frequency
$\nu_e$ at the frequency of the level splitting. The resulting dressed qubit
Rabi frequency $\Omega_{R\rho}$ is defined by the modulation amplitude, which
can be made comparable to the level splitting using frequency modulation on the
microwave source. This allows us to investigate the regime where the rotating
wave approximation breaks down, without requiring microwave power levels that
would be incompatible with a cryogenic environment. We observe clear deviations
from normal Rabi oscillations and can numerically simulate the time evolution
of the states in excellent agreement with the experimental data.
