  The interplay between quantum-mechanical and classical evolutions in a
chirped driven Rydberg atom is discussed. It is shown that the system allows
two continuing resonant excitation mechanisms, i.e., a successive two-level
transitions (ladder climbing) and a continuing classical-like nonlinear phase
locking (autoresonance). The persistent $1:1$ and $2:1$ resonances between the
driving and the Keplerian frequencies are studied in detail and characterized
in terms of two dimensionless parameters $P_{1,2}$ representing the driving
strength and the nonlinearity in the problem, respectively. The
quantum-mechanical rotating wave and the classical single resonance
approximations are used to describe the regimes of efficient classical or
quantum-mechanical excitation in this two-parameter space.
