In this study we examined the energy transfer dynamics of a FRET coupled pair of chromophores at the single molecule level embedded in a tunable sub-wavelength Fabry-Pérot resonator with two silver mirrors and separations in the λ/2 region. By varying the spectral mode density in the resonator via the mirror separation we altered the radiative relaxation properties of the single chromophores and thus the FRET efficiency. We were able to achieve wavelength dependent enhancement factors of up to three for the spontaneous emission rate of the chromophores while the quenching due to the metal surfaces was nearly constant. We could show by confocal spectroscopy, time correlated single photon counting and time domain rate equation modeling that the FRET rate constant is not altered by our resonator.