Wayde van Niekerk of South Africa (C) crosses the finish line ahead of Luguelin Santos of the Dominican Republic and Kirani James of Grenada (R) 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]

"It's a very special occasion. It's a once in a lifetime opportunity," he said. "I'm just glad to be part of this special group of 400 metres runners in this era.

"I just tried to do my best out here."

Van Niekerk holds both the South African 200 and 400 metres records but was unable to run in both in Beijing because of the schedule.

With the personal best of 19.94 he ran in the 200 metres earlier this year, he joined Michael Johnson, Merritt and Isaac Makwala as the only athletes to have gone under 20 seconds in the 200 and 44 seconds in the 400.

Only Johnson, who set the world record of 43.18 in 1999, and fellow Americans Butch Reynolds and Jeremy Wariner have gone faster over one lap.

Botswana's Makwala, who also joined the sub-44 club this year and had the best time of 2015 going into the final, finished fifth in 44.63.