From spin resonance of two-dimensional (2D) conduction electrons in a modulation doped SiGe/Si/SiGe quantum well structure we find a 2D anisotropy of both the line broadening (dephasing time) and the g factor. Both can be explained consistently employing the Bychkov-Rashba (BR) term H-BR=alpha(kxsigma).e(z), which turns out here to be the dominant coupling between electron orbital motion and spin. We obtain a BR parameter of alpha=0.55x10(-12) eV cm-three orders of magnitude smaller than in quantum well structures based on III-V semiconductors, consistent with the much smaller spin-orbit coupling in Si.