We demonstrate a 1.4W continuous-wave (CW) laser at 243.1nm. The radiation is generated through frequency quadrupling the output of a ytterbium-doped fiber amplifier system. which produces >10 W of CW power at 972.5nm. We demonstrate absolute frequency control by locking the laser to an optical frequency comb and exciting the 1S-2S transition in atomic hydrogen. This frequency-stabilized, high-power deep-UV laser is of significant interest for precision spectroscopy of simple and exotic atoms, two-photon laser cooling of hydrogen, and Raman spectroscopy.