The 24-year-old Van Niekerk, who is the reigning World and Olympic champion in the 400m, clocked a winning time of 19.84 on a temporary built track to beat runner-up BeeJay Lee (20.17) at the Boston Boost Games.
Because the competition was run on a straight line instead of the traditional curved track and field surface the time will not be listed in 2017 International Association of Athletics Federation (IAAF) records.
"I still have a lot of work ahead of me," said Van Niekerk, who will run the 200m next weekend in Kingston, Jamaica. "But it is a positive result."
