The origin of terahertz (THz) generation in a gas-phase medium is still in controversy, although the THz sources have been applied across many disciplines. Herein, the THz generation in a dual-color field is investigated experimentally by precisely controlling the relative phase and polarization of dual-color lasers, where the accompanying third-harmonic generation is employed for in situ determination of the relative phase up to sub-wavelength accuracy. Joint studies with the strong approximation (SFA) theory reveal that the continuum-continuum (CC) transition within an escaped electron wave packet in the single atom gives birth to THz emission, without the necessity of considering the plasma effect. Meanwhile, we develop the analytic form from SFA-based CC description, which is able to reproduce and decompose the classical photocurrent model from the viewpoint of microscopic quantum theory, establishing the quantum-classical correspondence and bringing a novel insight into the mechanism of THz generation. Present studies leave open the possibility for probing the ultrafast dynamics of continuum electrons and a new dimension for the study of THz-related science and methodology.