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Newly available x-ray nanobeams in synchrotron radiation facilities open new research avenues in the nanosciences. However, a significant challenge is to efficiently concentrate, particularly for high-energy x-rays, a large photon flux into a very small focal spot. Here, we demonstrate for the first time, to the best of our knowledge, a sub-13-nm (FWHM) diffraction-limited x-ray focus size with 6 billion photons per second formed using elliptically figured mirrors operating at 33.6 keV. This is the smallest and brightest focus spot in this high-energy range. We report the results of an x-ray ptychography experiment to characterize the wavefront at the focus position. This offers new opportunities in multidisciplinary fields for x-ray microscopy techniques in which the focal spot size limits the resolution.