We introduce the Riesz operator in the context of Gromov hyperbolic groups in order to investigate a one parameter family of non unitary boundary Hilbertian representations of hyperbolic groups. We prove asymptotic Schur's relations, the latter being the main result of this paper. Up to normalization, the Riesz operator plays the role in the context of hyperbolic groups of the Knapp-Stein intertwiner for complementary series for Lie groups. Assuming the positivity of the Riesz operator, we define an analogue of complementary series for hyperbolic groups and prove their irreducibility.

Corrections have been done from the previous version: in particular the construction of the representation K'_{t} and the uniform bound dealing with H_t have been modified