Wayde Van Niekerk sets a world record in the 400-meter dash Sunday night. (Jonathan Newton/The Washington Post)

Wayde van Niekerk started out in Lane 8, wide to the right of two Olympic champions, the starting position usually reserved for runners who go home and beam with pride about just making the final. He could see none of his competitors in the men’s 400 meters, and no one would suspect him. He hailed from South Africa, which had not produced an individual sprinting gold medalist since 1920. He employed as a coach a 74-year-old, white-haired Namibian great-grandmother.

Van Niekerk was not a nobody, not by a long shot: He had run the fourth-fastest 400 time ever last year to win the world championships. But he put his feet in the blocks Sunday night as an afterthought in his own race, let alone in the entire program, on a night Usain Bolt would end his Olympic 100-meter career. Van Niekerk finished second to no one. On a warm night at Olympic Stadium, Wayde van Niekerk of Cape Town, South Africa, ran once around an oval track faster than any man ever had before.

In the span of 43.03 seconds, van Niekerk’s life changed, his coach became an instant Internet celebrity and the fabled record Michael Johnson held for 17 years fell by a margin of 0.15 seconds. When van Niekerk crossed the finish line ahead of silver medalist Kirani James of Grenada and American bronze winner LaShawn Merritt — the gold medalists in 2012 and 2008, respectively — he stared at the race clock, wiped his face and fell to his knees, barely able to process what he had done.

“To be honest with you,” van Niekerk said. “I never envisioned I would set the world record.”

No athlete could steal the spotlight from Bolt as he ran the final 100-meter race of his career. Van Niekerk had at least managed to rent space under it. He went from unknown outside track circles to Bolt’s global co-star.

“My favorite event is the 200 meters,” van Niekerk said, chuckling. “I’d love to race him.”

Van Niekerk’s ascension began three years ago, when he started working under a 74-year-old great-grandmother named Ans Botha . She is a renowned figure in South African track, but van Niekerk introduced to her a world stage, thanks largely to a well-timed Daily Mail story that pinballed around social media Sunday night. Her runners call her Tannie Ans: Aunt Ans in her native Afrikaans.

Botha may seem like an oddity, only a comic contrast for a world-class sprinter. But among her runners, her instruction is serious and sacred.

“When I first met her, I was afraid of her,” said South African sprinter Akani Simbine, van Niekerk’s roommate in Rio. “I was scared of her. She seems very strict. But she was very loving. As soon as she got to know me, she started showing love.

“She’s very loving, you know. But when the track workouts need to be done, she makes sure you get them done. She makes sure that you get the message. But she gets excited. When gets really excited, she lets out a big, ‘Yes!’ ”

Botha may have let out the biggest one yet Sunday night. Van Niekerk started in Lane 8 after running a semifinal qualifying time of 44.45 seconds. He also runs the 200 meters, so he possesses the ability to start fast. Midway through the race, he had seized control.

University of Virginia physics professor Lou Bloomfield explains some of the fundamental forces at work in Olympic sprinting, and how runners use sprinting blocks to get ahead. (Thomas Johnson,Julio Negron,Danielle Kunitz,Dani Johnson,Osman Malik/The Washington Post)

“When a guy goes out like that, the last 100, usually he slows down a little bit,” James said. “But he just kept going. And when you keep going like, a world record is going to fall.

In Lanes 5 and 6, Merritt and James jockeyed around the final turn. They both saw van Niekerk to their right, a blur up ahead. They both thought he would fade. His lead only grew more impenetrable, lithe and strong, leaning for the finish.

“I thought somebody was going to catch me,” van Niekerk said. “I felt very alone at the end. I thought, ‘What’s going on?’ ”

“He was running blind, so he really didn’t have anybody to cue off of,” Merritt said. “It was just, when the gun goes off, take off. And that’s what gets you 43.0.”

The race served as a leap forward not only for van Niekerk, but for the 400 itself. Merritt’s bronze-winning time of 43.85 would have won gold in 2012 and fell just shy of the 43.75 he ran to win gold at the Beijing 2008 Games.

“The event is getting stronger,” Merritt said. “I knew the record was going to go soon. And the 43.0 will go one day as well. It’s just a great era for the 400.”

Merritt’s bronze served as a sweet accomplishment. In 2008, just 22 and coming off two gold medals in Beijing, Merritt owned a career’s worth of accomplishment and much more time to add to it.

The wave of euphoria lasted less than 18 months. Between October 2009 and January 2010, Merritt tested positive three times for banned substances DHEA and pregnenolone. Merritt publicly insisted the positive tests resulted from frequent usage of ExtenZe, an herbal sexual enhancement product sold at gas stations and convenience stores. He appealed his case before an arbiter, who determined Merritt had not intentionally doped, so his name was cleared. He still received a 21-month suspension, blamed for negligence.

In May 2012, by which time Merritt had reestablished himself, Merritt tore a hamstring. He still qualified for London, but the injury forced him to pull out of the Games after running one preliminary heat. Before Sunday, Merritt again had reclaimed his place among the world’s best, and he viewed his Olympic return with little introspection.

“Another medal,” Merritt said. “I had world championships in there with the same guys on top of the podium. This is the Olympic Games, it’s a little bigger stage. It’s still the same guys who go out battle, and I’m happy to get back here on this stage.”

The bronze did not strike Merritt as a culmination. He believes he will compete in Tokyo in four years, at age 34.

“My body feels good,” Merritt said. “I still love the sport. I’ll continue to do it as long as I’m enjoying.”

In a sense, Merritt strode to the starting line Sunday night still defending a medal eight years after he won it: No one had yet beaten him in an Olympics he could compete in.

But he had never competed against van Niekerk at the Games. No one had ever competed against a race like he ran Sunday. James hugged van Niekerk, happy to be part of history. In that moment, and many to follow, van Niekerk was still attempting to wrap his mind around it.

“At this moment,” van Niekerk said, “I’m just grateful for everything that happened tonight.”