We demonstrate high-fidelity manipulation of the quantized motion of a single 87 Rb atom in an optical tweezer via microwave couplings induced by Ste-Gerlach splitting.The Ste-Gerlach splitting is mediated by polarization gradient of a strongly focused tweezer beam that functions as fictitious magnetic field gradient.The spatial splitting removes the orthogonality of the atomic spatial wavefunctions,thus enables the microwave couplings between the motional states.We obtain coherent Rabi oscillations for up to third-order sideband transitions,in which a high fidelity of larger than 0.99 is obtained for the spin-flip transition on the first order sideband after subtraction of the state preparation and detection error.The Ste-Gerlach splitting is measured at a precision of better than 0.05 nm,This work paves the way for quantum engineering of motional states of single atoms,and may have wide applications in few body physics and ultracold chemistry.