Wayde van Niekerk of South Africa (R) runs past Kirani James of Grenada (L) and Isaac Makwala of Botswana to win the men's 400 metres final at the IAAF World Championships at the National Stadium in Beijing, China August 26, 2015. [Photo/Agencies]

BEIJING - Wayde van Niekerk ran the sixth fastest 400 metres of all time to beat a stellar field and become the first South African to win a world championship sprint title on Wednesday.

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.

The South African collapsed onto the track exhausted at the end of the race and was taken straight off for medical treatment.

Olympic champion Kirani James of Grenada finished third to claim bronze in 43.78, his best run of the year, ensuring the top three ran under 44 seconds for the first time at a world championships final.

"It's crazy. We're warriors. We're animals. The 400 is never easy. We'll rise to the occasion. I love this. I love this sport," said Merritt.

"I ran well, it wasn't enough for the gold tonight, but the winning time was a great time I can't complain. Got the silver and can go home with some hardware."

James, one of a record five runners in the field who had a personal best under 44 seconds, said he thought he had a chance coming into the final straight but the pace was just too hot for him.