  Precise control of the electron phase accumulated during its sub-cycle motion
within intense laser fields is essential in strong-field physics, yet remains
mostly indirect and complicated so far. In this Letter, we develop a novel
approach to control this sub-cycle electron phase by tuning a low-frequency
electric field applied on a centrosymmetric gaseous target during its
interaction with a few-cycle infrared laser pulse. Our method is based on a
universal analytical relation between the low-frequency electric field and its
induced harmonic frequency shift, derived by the strong-field approximation.
This simple relation and its universality are confirmed numerically by directly
solving the time-dependent Schr\"odinger equation. Moreover, we discuss the
benefits of the discovered relation in \textit{in situ} applications, including
continuously and precisely tuning XUV waves and developing a new method of
comprehensively sampling THz pulse.
