Robust identification with FIR models fails to be successful when the number of coefficients to be estimated becomes large, caused by Lightly damped modes of the plant or poles very close to the unit circle. The paper presents a two-stage algorithm to obtain a low-order approximate model in frequency domain in a generalized orthonormal basis with guaranteed H-infinity error bound for deterministic linear time-invariant stable systems, the first stage being an L-infinity rational approximation and the second nonlinear step being an H-infinity rational approximation.