Leaky surface modes on an ultra-thin anisotropic gold metasurface are directly observed through scattering-type scanning near-field optical microscopy (s-SNOM). Surface modes are excited by focussing light onto a resonant silicon nano-sphere positioned on the surface by nano-manipulation under electron-beammonitoring. The light scattered from the nano-sphere is ableto excitesurface modes with a broad range of wavevectors. The asymmetric air-metasurface-insulator structure supports a leaky surface mode which is confined at the top interface and leaks into the higher-index substrate. s-SNOM near-field mapping of the leaky surface wave is in good agreement with the results of the full-wave simulationsof the electro-magnetic modes on the structure.