Accurate evaluation of the non-linear wave-wave energy transfer requires a significant proportion of the computation time of ocean wave models. The Discrete Interaction Approximation (DIA) developed within the first version of the WAM model (WAMDI, 1988) is the only algorithm to be used today in operational wave modelling as it is the only way to calculate the wave-wave interactions rapidly enough. In this study, the Two- Scale Approximation (TSA), a potential successor method to the DIA, was successfully implemented in the third generation operational wave model WAVEWATCH IIITM(WW3). Preliminary results (Perrie and Resio, 2009) showing that it offers improved accuracy are confirmed in this study by the modelling of wave evolution under constant winds. Fetch-growth curves and two-dimensional spectra for energy and non-linear wave-wave interactions obtained using the TSA in these conditions show better agreement to more exact computations of non-linear interactions, than the DIA results.