The discovery of luminous quasars at redshift z ~ 6 indicates the presence of supermassive black holes (SMBHs) of mass ~10⁹ M when the universe was less than 1 billion years old. This finding presents several challenges for theoretical models because whether such massive objects can form so early in the ΛCDM cosmology, the leading theory for cosmic structure formation, is an open question. Furthermore, whether the formation process requires exotic physics such as super-Eddington accretion remains undecided. Here we present the first multiscale simulations that, together with a self-regulated model for the SMBH growth, produce a luminous quasar at z ~ 6.5 in the ΛCDM paradigm. We follow the hierarchical assembly history of the most massive halo in a ~3 Gpc³ volume and find that this halo of ~8 × 10¹² M forming at z ~ 6.5 after several major mergers is able to reproduce a number of observed properties of SDSS J1148+5251, the most distant quasar detected at z = 6.42 (Fan et al. 2003). Moreover, the SMBHs grow through gas accretion below the Eddington limit in a self-regulated manner owing to feedback. We find that the progenitors experience vigorous star formation (up to 10⁴ M yr^-1) preceding the major quasar phase such that the stellar mass of the quasar host reaches 10¹² M at z ~ 6.5, consistent with observations of significant metal enrichment in SDSS J1148+5251. The merger remnant thus obeys a similar M_BH-M_bulge scaling relation observed locally as a consequence of coeval growth and evolution of the SMBH and its host galaxy. Our results provide a viable formation mechanism for z ~ 6 quasars in the standard ΛCDM cosmology and demonstrate a common, merger-driven origin for the rarest quasars and the fundamental M_BH-M_bulge correlation in a hierarchical universe.