American Johnson was dumbfounded by the quality of Van Niekerk 's finish .
American Johnson won back - to - back Olympic titles in the event in 1996 and 2000 and is considered one of the greatest sprinters of all time .
American Johnson , who won back - to - back Olympic titles in the event in 1996 and 2000 and is considered one of the greatest sprinters of all time , was dumbfounded by the quality of Van Niekerk 's finish .