Consider the anisotropic Navier-Stokes equations as well as the primitive equations. It is shown that the horizontal velocity of the solution to the anisotropic Navier-Stokes equations in a cylindrical domain of height ε with initial data u 0 = (v 0 , w 0) ∈ B 2−2/p q,p , 1/q + 1/p ≤ 1 if q ≥ 2 and 4/3q +2/3p ≤ 1 if q ≤ 2, converges as ε → 0 with convergence rate O(ε) to the horizontal velocity of the solution to the primitive equations with initial data v 0 with respect to the maximal-L p-L q-regularity norm. Since the difference of the corresponding vertical velocities remains bounded with respect to that norm, the convergence result yields a rigorous justification of the hydrostatic approximation in the primitive equations in this setting. It generalizes in particular a result by Li and Titi for the L 2-L 2-setting. The approach presented here does not rely on second order energy estimates but on maximal L p-L q-estimates for the heat equation.