Du Plessis' amateur record dates from 1948, when he fought for two years as a junior at weights ranging from 114 and 135 pounds.

On 17 April 1952, Du Plessis lost to Theuns van Schalkwyk on points during the final of the South African trials for the 1952 Olympic Games.

Du Plessis finished his amateur career on 27 September 1952, with a first-round knockout of A Nel in Bloemfontein, amassing a record of 30-19-3 (16 KO).

After his first four bouts, and victories over Corrie van Niekerk and Alex Weiss,[2] Du Plessis was ranked as Class A by the magazine Boxing Round-Up.

[3] After fighting 13 more times during the next three years, Du Plessis earned a title shot against reigning South African middleweight champion, Mike Holt.

Du Plessis fought Mike Holt for the South African Middleweight title in the Durban City Hall on 12 September 1956.