We report the efficient generation of 100-W single-frequency radiation with diffraction-limited beam quality at the 1064-nm wavelength by use of a master-oscillator fiber power-amplifier system, consisting of a diode-pumped monolithic nonplanar ring laser as the master oscillator and an Yb-doped large-mode-area fiber as the power amplifier. The emission spectrum, the intensity noise behavior, and further power-scaling possibilities to the >200-W level, which are determined by the threshold of stimulated Brillouin scattering in the fiber amplifier, are discussed.