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There are four massive quasars discovered at z>5. These sources present a puzzle how black holes grow to 10elev9 solar masses in the limited age of the universe. We would like to address this by estimating the population of such objects. The population estimation requires good constraints on their jet properties comma in particular comma the Doppler factor. We propose to observe the high-redshift blazar SDSSJ013127.34-032100.1 with XMM and NuSTAR for 40 ks and 100 ks comma respectively comma in order to model the broadband SED accurately. The broadband X-ray observation will allow us to measure the Doppler factor for the source comma constraining the high-a supermassive BH population in the early universe. These measurements will be used to infer the number of similar BHs at high redshifts.