We employ cosmological hydrodynamical simulations to study the growth of massive black holes ( BHs ) at high redshifts subject to BH merger recoils from gravitational wave emission . As a promising host system of a powerful high-redshift quasar , we select the most massive dark matter halo at z = 6 from the Millennium simulation , and resimulate its formation at much higher resolution including gas physics and a model for BH seeding , growth and feedback . Assuming that the initial BH seeds are relatively massive , of the order of 10 ^ { 5 } { M } _ { \odot } , and that seeding occurs around z \sim 15 in dark matter haloes of mass \sim 10 ^ { 9 } -10 ^ { 10 } { M } _ { \odot } , we find that it is possible to build up supermassive BHs ( SMBHs ) by z = 6 that assemble most of their mass during extended Eddington-limited accretion periods . The properties of the simulated SMBHs are consistent with observations of z = 6 quasars in terms of the estimated BH masses and bolometric luminosities , the amount of star formation occurring within the host halo , and the presence of highly enriched gas in the innermost regions of the host galaxy . After a peak in the BH accretion rate at z = 6 , the most massive BH has become sufficiently massive for the growth to enter into a much slower phase of feedback-regulated accretion . We extend our basic BH model by incorporating prescriptions for the BH recoils caused by gravitational wave emission during BH merger events , taking into account the newest numerical relativity simulations of merging BH binaries . In order to explore the full range of expected recoils and radiative efficiencies we also consider models with spinning BHs . In the most ‘ pessimistic ’ case where BH spins are initially high , we find that the growth of the SMBHs can be potentially hampered if they grow mostly in isolation and experience only a small number of mergers . On the other hand , whereas BH kicks can expel a substantial fraction of low mass BHs , they do not significantly affect the build up of the SMBHs . On the contrary , a large number of BH mergers has beneficial consequences for the growth of the SMBHs by considerably reducing their spin . We also track the fate of our z = 6 SMBH by performing cosmological simulations all the way to z = 2 . This allows us to study the history of BH mass assembly over a large time-span and to establish a clear signal of ‘ downsizing ’ of the BH accretion rate for the population of BHs as a whole . We further find that the descendents of the most luminous z = 6 quasar correspond most likely to the most massive BHs today , characterized by a low activity level and masses of the order of 1 - 2 \times 10 ^ { 10 } { M } _ { \odot } .