  The radio-loud quasar SDSS J102623.61+254259.5, at a redshift z=5.3, is one
of the most distant radio-loud objects. Since its radio flux exceeds 100 mJy at
a few GHz, it is also one of the most powerful radio-loud sources. We propose
that this source is a blazar, i.e. we are seeing its jet at a small viewing
angle. This claim is based on the spectral energy distribution of this source,
and especially on its strong and hard X-ray spectrum, as seen by Swift, very
typical of powerful blazars. Observations by the Gamma-Ray Burst
Optical/Near-Infrared Detector (GROND) and by theWide-field Infrared Survey
Explorer (WISE) allow to establish the thermal nature of the emission in the
near IR-optical band. Assuming that this is produced by a standard accretion
disk, we derive that it emits a luminosity of L_d \simeq 9 \times 10^46 erg
s^{-1} and that the black hole has a mass between 2 and 5 billion solar masses.
This poses interesting constraints on the mass function of heavy (> 10^9 M_sun)
black holes at high redshifts.
