Referee Lou Moret waved the fight off at 1:53 of the ninth round after Mchunu had been down for the third time, Joinfo.com reports with reference to Supersport.
Usyk, the 2012 Olympic heavyweight gold medalist, started slowly against the defensive style and awkward spoiling tactics of Mchunu.
However, Mchunu did score with counter left hands in this all southpaw battle, but Usyk began to land the more accurate punches as the bout progressed.
Mchunu was put down with a left uppercut in the sixth round. He was sent down again in round nine with a flurry of punches before Usyk scored the final knockdown later in the round with a left hook.
Usyk improves his record to 11-0; 10 and Mchunu’s record drops to 17-3; 12.
Watch the match from Ikick on Youtube: