  Active galactic nuclei (AGN) have been observed as far as redshift z~7. They
are crucial in investigating the early Universe as well as the growth of
supermassive black holes at their centres. Radio-loud AGN with their jets seen
at a small viewing angle are called blazars and show relativistic boosting of
their emission. Thus, their apparently brighter jets are easier to detect in
the high-redshift Universe. DES J014132.4-542749.9 is a radio-luminous but
X-ray weak blazar candidate at z = 5. We conducted high-resolution radio
interferometric observations of this source with the Australian Long Baseline
Array at 1.7 and 8.5 GHz. A single, compact radio emitting feature was detected
at both frequencies with a flat radio spectrum. We derived the
milliarcsecond-level accurate position of the object. The frequency dependence
of its brightness temperature is similar to that of blazar sources observed at
lower redshifts. Based on our observations, we can confirm its blazar nature.
We compared its radio properties with those of two other similarly X-ray-weak
and radio-bright AGN, and found that they show very different relativistic
boosting characteristics.
