This paper is devoted to the control of bilateral teleoperation systems when the nonlinear external environment forces are considered as measured perturbations. To deal with this, we propose a nonlinear control strategy based on the pi-flatness concept which is an interesting property for the tracking control of nonlinear time-delays systems. This strategy is composed by two steps: an open-loop feedforward control which allows to track the reference master trajectory; and a stabilization of the slave system around the desired trajectory using a predictive polynomial control. Therefore, we extend the usual Generalized Predictive Control (GPC) via a pi-flat output to the case where the reference master trajectory is not a priori known. This difficulty is due to the bilateral aspect where the environment force feedback alters this reference trajectory. This extended GPC structure introduces robustness properties which are discussed in this paper. An experimental validation of the proposed approach is presented between two remote sites 1000 km away through Internet.