We propose a scheme for infrared single-photon detection based on two-photon absorption at room temperature in Si avalanche photodiodes, where the detected photon's energy is lower than the band gap and the energy difference is complemented by a pump field. A quantum nonperturbative model is developed for nondegenerate two-photon absorption in direct and indirect semiconductors yielding proper nondivergent rates allowing device efficiency optimization. The proposed monolithic detector is simple, miniature, and integrable and does not require phase matching, while not compromising the performance and exhibiting even better efficiency than the competing up-conversion schemes ($\ensuremath{\sim}1$ order of magnitude) for similar optical pump levels.