  It is well known that for flat systems the tracking control problem can be
solved by utilizing a linearizing quasi-static feedback of generalized states.
If measurements (or estimates) of a so-called generalized Brunovsk\'y state are
available, a linear, decoupled and asymptotically stable tracking error
dynamics can be achieved. However, from a practical point of view, it is often
desirable to achieve the same tracking error dynamics by feedback of a
classical state instead of a generalized one. This is due to the fact that the
components of a classical state typically correspond to measurable physical
quantities, whereas a generalized Brunovsk\'y state often contains higher order
time derivatives of the (fictitious) flat output which are not directly
accessible by measurements. In this paper, a systematic solution for the
tracking control problem based on quasi-static feedback and measurements of
classical states only is derived for the subclass of $(x,u)$-flat systems.
