Wayde van Niekerk of South Africa, en route to setting a world record in the 400m on Sunday. [Photo/Agencies]

Win or lose, Usain Bolt was always going to dominate the Olympic headlines on Sunday.

But South Africa's Wayde van Niekerk offered his embattled sport some hope for after the Jamaican's retirement with a stunning world record on the Rio track.

Twenty minutes before Bolt won his third straight 100m title, van Niekerk ran at an unrelenting pace to clinch the gold medal and break Michael Johnson's 17-year-old record of 43.03 sec in the 400m.

It was the men's oldest sprint record - Bolt owns the 100m and 200m marks, - and one many thought might never be eclipsed.

"I believed it was possible," said the 24-year-old Niekerk.

"I am just glad things went my way tonight. I'm still a bit amazed. I still have to pinch myself over what just happened."

His achievement was all the more remarkable because he ran in lane eight, the outside lane where it is impossible to see any of your rivals.

"I don't think any athlete wants to be in lane eight, but I figured it has advantages as well as disadvantages," he said.

"You have the perfect opportunity to go out as if you are in training and go as hard as you can."

Van Niekerk has been training with Bolt's team this year and the Jamaican, who has said he will retire after the world championships in London next year, ran over to congratulate him after finishing the 100m.

"When he got the world record I was like, wow!" said Bolt, who gave up the 400m because he did not like the training.

"I told him in Jamaica that my coach said he's probably the only guy right now other than me who can break this 400m world record. I'm really happy for him, I'm really proud of him."

Van Niekerk said he was just happy to be at his first Olympics alongside the likes of Bolt, describing him as the "king of the sprints".

"I'm just really grateful that I'm part of this generation of athletics, just keeping the sport alive," he said.

"I use these guys as inspiration and motivation, but I've now got a chance to build my own legacy, my own journey and story. Hopefully, I can inspire other South Africans and athletes around the world to go out and do their thing."

Van Niekerk had a straightforward message for anyone who suggested his remarkable progress in the past four years was down to doping.

"I know I'm not, so what else can I say on that?" he said.

Two-time Olympic champion Johnson was dumbfounded by the quality of Van Niekerk's run.

"Van Niekerk is so young, what else can he do? Can he go under 43 seconds? It is something I thought I could do, but never did," the American said.

"Usain Bolt will be retiring soon, so this guy could be the next star."