The observation of quasars at very high redshift such as Pōniuā'ena is a challenge for models of supermassive black hole (SMBH) formation. This work presents a study of SMBH formation via known physical processes in starburst clusters formed at the onset of the formation of their hosting galaxy. While at the early stages hypermassive starburst clusters reach the luminosities of quasars, once their massive stars die, the ensuing gas accretion from the still forming host galaxy compresses its stellar black hole (BH) component to a compact state overcoming heating from the BH-BH binaries such that the cluster collapses, forming a massive SMBH-seed within about a hundred Myr. Within this scenario, the SMBH-spheroid correlation emerges near to exactly. The highest redshift quasars may thus be hypermassive starburst clusters or young ultracompact dwarf galaxies (UCDs), being the precursors of the SMBHs that form therein within about 200 Myr of the first stars. For spheroid masses ${\lesssim }10^{9.6}\, \mathrm{M}_\odot$ , an SMBH cannot form and instead only the accumulated nuclear cluster remains. The number evolution of the quasar phases with redshift is calculated and the possible problem of missing quasars at very high redshift is raised. SMBH-bearing UCDs and the formation of spheroids are discussed critically in view of the high-redshift observations. A possible tension is found between the high star formation rates (SFRs) implied by downsizing and the observed SFRs, which may be alleviated within the IGIMF theory and if the downsizing times are somewhat longer.