We study theoretically the behavior of two atomic Rydberg series, coupled by a light field that strongly drives the optical transition between the core states to which these series converge. It is shown that this strong-field version of isolated-core excitation leads to a strong modification of the photoionization spectrum and to transitions between adjacent Rydberg states when the Rabi frequency associated with the core transition becomes larger than the Rydberg spacing. The intensities and pulse durations (ª 2 GW/cm2 and 1 ps, respectively, for n ª 20 Rydberg states) needed for the observation of these effects are within reach of present-day lasers.