The 23-year-old went hard from the start and held off reigning champion La Shawn Merritt of the United States down the home straight to claim the title in 43.48 seconds.
A personal best of 43.65 was worth no better than silver for Merritt, who stayed on the South African's shoulder around the final bend but could not find enough gas in the last 50 metres to overhaul him.
Olympic champion Kirani James of Grenada finished third to claim bronze in 43.78, his best run of the year.
(Reporting by Nick Mulvenney, editing by Mitch Phillips)
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