The optical response of metal surfaces to high-power laser excitation (HPLE) is one of important topics of laser-matter interaction, which remains to be poorly understood. It is known that certain metals, which are well reflective at normal conditions, become considerably absorbing during irradiation by ultrashort high-power lasers. Reflectivity of such metals during the irradiation stage is a key parameter of the HPLE process as it determines the absorbed laser energy and, hence, the post-irradiation evolution of the material. In this paper, we present a two-temperature model of femtosecond laser irradiation of zinc, which takes into account the dynamic change of surface reflectivity due to excitation of the conduction electrons. The model involves integration of the reflected part of the laser beam over the irradiation spot to enable comparisons with experimental data. The numerical scheme which combines the description of the reflectivity change with the TTM is discussed in detail.