  We give a new proof of Witten asymptotic conjecture for Seifert manifolds
with non vanishing Euler class and one exceptional fiber. Our method is based
on semiclassical analysis on a two dimensional phase space torus. We prove that
the Witten-Reshetikhin-Turaev invariant of a Seifert manifold is the scalar
product of two Lagrangian states, and we estimate this scalar product in the
large level limit. The leading order terms of the expansion are naturally given
in terms of character varieties, the Chern-Simons invariants and some
symplectic volumes. For the analytic part, we establish some singular
stationary phase lemma for discrete oscillatory sums.
