We study the behavior of invariant sets of a volume-preserving map
that is a quasiperiodic perturbation of a symplectic map,
using approximation by periodic orbits. We present numerical
results for analyticity domains of invariant surfaces, behavior after
breakdown, and a critical function describing breakdown of invariant surfaces
as a function of their rotation vectors. We discuss implications
of our results to
the existence of a renormalization group operator describing breakdown
of invariant surfaces.