Wayde van Niekerk ran the fastest 400 metres since 2007 to become the first South African to win a world championship sprint title on Wednesday.
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.
South Africa's Van Niekerk stuns champions to win 400 gold
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.
Olympic champion Kirani James of Grenada finished third to claim bronze in 43.78, his best run of the year.
