We report 6yr monitoring of distant bright quasar CTS C30.10 (z=0.90052) with the Southern African Large Telescope. We measured the rest-frame time lag of 562_-68_^+116^ days between the continuum variations and the response of the MgII emission line, using six different methods. This time delay, combined with other available measurements of MgII line delay, mostly for lower-redshift sources, shows that the MgII line reverberation implies a radius-luminosity relation very similar to the one based on a more frequently studied H{beta} line.