Nonlocal correlation effects in the half-filled Hubbard model on an isotropic triangular lattice are studied within a spin polarized extension of the dual fermion approach. A competition between the antiferromagnetic noncolinear and the spin-liquid states is strongly enhanced by incorporation of a k-dependent self-energy beyond the local dynamical mean-field theory. The dual fermion corrections drastically decrease the energy of a spin-liquid state while leaving the noncolinear magnetic states almost nonaffected. This makes the spin liquid a preferable state in a certain interval of interaction strength of an order of magnitude of a bandwidth. The spectral function of the spin-liquid Mott insulator is determined by a formation of local singlets which results in an energy gap about two times larger than that of the 120 degrees antiferromagnetic Neel state.