Unexpected formation of regular high-Q whispering gallery modes in a deformed microdisk where the radius is of the order of the vacuum wavelength is explained in terms of partial barriers in phase space. Using a semiclassical approach to determine the action flux of the partial barriers, we successfully predict spectral ranges in which the high-Q modes can exist. Our analysis enables optimization of emission directionality and the Q factor of deformed ultrasmall microcavities.