Multi-scale simulations of gas-rich major mergers between massive high-z disk galaxies have shown that gas inflows can be triggered that damp mass in the cores at more than a 1000 Mo/yr, forming nuclear supermassive disks (SMDs). We report on the first cosmological simulations that follow a merger between two early forming massive disk galaxies, comparable to the present-day Milky Way, occurring at redshift just below 8, which combine the zoom-in technique and particle splitting to reach pc scale resolution. An SMD only 4 pc in size forms which has super-solar metallicity due to the highly biased environment, yet it hardly fragments due to the high turbulence of the ISM. The SMD is self-gravitating and bar-unstable We use an analytical model to study the later evolution of the SMD due to internal transport of angular momentum resulting from the bar instability, showing that it reaches the conditions for the general relativistic radial instability in less than a million years, forming a supermassive black hole with mass up to 100 million solar masses. This mechanism skips the stage of BH seed formation, and provides a natural explanation for the bright high-z QSOs.