We have characterized the x-ray phase vortices generated at the focal spot of various spiral Fresnel zone plates with an outermost zone width of Δr = 50 nm. The complex-valued wavefields of phase vortices as small as 50 nm in size (FWHM) and several topological charges were reconstructed using ptychographic coherent diffractive imaging. The reconstructed focal spots demonstrate good agreement with the theoretically expected wavefields and diffractionlimited focusing.