  The relaxation dynamics of the contact angle between a viscous liquid and a
smooth substrate is studied at the nanoscale. Through atomic force microscopy
measurements of polystyrene nanostripes we monitor simultaneously the temporal
evolution of the liquid-air interface as well as the position of the contact
line. The initial configuration exhibits high curvature gradients and a
non-equilibrium contact angle that drive liquid flow. Both these conditions are
relaxed to achieve the final state, leading to three successive regimes along
time: i) stationary-contact-line levelling; ii) receding-contact-line
dewetting; iii) collapse of the two fronts. For the first regime, we reveal the
existence of a self-similar evolution of the liquid interface, which is in
excellent agreement with numerical calculations from a lubrication model. For
different liquid viscosities and film thicknesses we provide evidence for a
transition to dewetting featuring a universal critical contact angle and
dimensionless time.
