South Africa's Wayde van Niekerk smashes Michael Johnson's record to claim 400m gold

• South African beats Kirani James and LaShawn Merritt to win 400m
• Van Niekerk’s time of 43.03sec beats Johnson’s 17-year world record

Wayde van Niekerk of South Africa celebrates after setting a new world record to win the men’s 400m. Photograph: Cameron Spencer/Getty Images

Wayde van Niekerk is a special, special athlete, the first ever to have run under 10 seconds for the 100m, 20 seconds for 200m and 44 for 400m. Now make that last a damn-near sub-43.

In front of a boisterous Rio crowd, Van Niekerk broke the first men’s track and field world record of these Games with a blistering, barely believable 43.03sec from the unlikely position of lane eight, destroying the two previous Olympic champions in the process.

Michael Johnson’s towering mark of 43.18sec had stood since 1999 and rarely been seriously challenged. Van Niekerk had kept his powder drier than most in qualifying here, thus is draw in lane eight, but when the gun went he rocketed from the blocks.

It looked, of course, as if he had gone off stupidly fast through the first 200m, and the expectation was that Kirani James, the defending champion from Grenada, and LaShawn Merritt of the United States – who fought like pitbulls in the middle lanes – would haul Van Niekerk in through the bends.

It didn’t happen. The South African in fact went faster in the second half of the race, slingshotting into the home straight in a blur of limbs. His winning margin against one of the fastest 400m fields ever assembled verged on the ridiculous. It was, as Johnson said on television commentary, “a massacre”.

“I believed I could get the world record. I’ve dreamed of this medal forever,” said Van Niekerk. “I am blessed.”

His rivals were somewhere between stunned and aghast afterwards. “I knew the time was going to be fast, [but] I didn’t think it was going to be 43.0 fast. It is what it is. You take it,” said Merritt. “He ran his heart out.”

James, a wonderful champion at London 2012 who might have been expected to dominate the distance for years, was incredulous at such a performance from the outside lane. “I was thinking the race would be around 43-mid,” he said. “He really couldn’t see anybody, I wasn’t sure how he was going to run his race out of lane eight, but he just kept going. He wouldn’t slow down.”

James referenced the performance of a fellow Grenadian in the Olympic final of 12 years past to attempt a benchmark. “The fastest Olympic time out of lane eight was Alleyne Francique in 2004 [44.66sec], so for him to come in and do that, a second and a half faster … I think that’s just incredible.”

After Van Niekerk’s triumph, one reporter asked him whether people can trust his record. “You can’t be anyone’s favourite. What I can do is control the controllables, and stay as disciplined as I can be, and focused on goals and life,” the South African replied. “The beginning of the year I had the opportunity to break the sub-10, as I believe the talent God has blessed me.”

Another reporter said he hasn’t answered the question. What do you say to people who say you are on drugs, the reporter asked. “I know I am not,” Van Niekerk replied.

Van Nierkerk is coached by a 74-year-old great-grandmother, Ans Botha: “She’s an amazing woman. She has played a huge role in who I am today and kept me very disciplined and very focused on the role and who I need to be. I’m very grateful my coach has pushed me to the limit. Anything is possible. I’m just grateful I can trust in her work.”

Van Niekerk is a 24-year-old from Cape Town, and he carried South Africa’s flag at the opening ceremony. He is not exactly a new face, having won last year’s world championship in Beijing, but of the three fastest men here was probably considered least likely for gold. Underestimating him is not a mistake that will be repeated.

Elsewhere on Sunday night, Laura Muir stepped into the 1500m semi-final as if to the manor born but soon found a harsher reality.

Muir, who announced herself on the world stage by breaking Kelly Holmes’ UK record at London’s Diamond League last month, loves to front-run, and took the race by the hair after half a lap, setting a strong pace right up to the bell. Then came an ambush: first Genzebe Dibaba of Ethiopia then Sifan Hassan of the Netherlands surged past her with a vicious injection of pace. Muir looked caught on the hop but she is no-one’s mug for long. She slowly began to reel the pair in and by the final bends was in touch, but not close enough to attempt a pass on the last. Dibaba and Hassan had played their cards correctly and held on down the home straight for all three to qualify comfortably, Muir in 4:04.16. USA’s Jennifer Simpson also made the final, finishing a place behind Muir.

In the other semi-final, GB’s Laura Weightman squeezed through in what was a faster, if less tactically cut-throat, affair. Kenya’s roughhousing champion Faith Kipyegon took no prisoners, winning in 4:03.95 and Weightman was lucky to hold the last qualifying position by a hundredth of a second back in fifth. USA’s Shannon Rowbury came home in third.

Christine Ohuruogu made what was surely her last Olympic appearance in the individual events, bowing out of the 400m at the semi-final stage in a sluggish 51.21sec. There was a familiar deficit to make up emerging from the final bends, but none of the old turbo-charge down the home straight, so memorable from her gold in Beijing eight years ago and silver at London 2012. With only the first two qualifying automatically, she was never in the picture and trailed home in fifth behind the winner Phyllis Francis of the US, who qualified in 50.31sec.

Emily Diamond, the other Briton to make it this far, had blamed food poisoning for scraping through the heats and it landed her a rough draw on the inside lane. She battled with vigour, but couldn’t quite beat the bends, finishing sixth in a semi-final won by Jamaica’s Shericka Jackson in a nippy personal best of 49.83sec.

Allyson Felix, the 2012 200m winner, strolled home in the final 400m semi, pulling up late for 49.67sec. There is an air of slight desolation about this event following the decision of Caster Semenya not to double up with the 800m, and the gold medal looks Felix’s for the taking. Shaunae Miller of the Bahamas, the only woman to run faster than Felix this year, looked a well-beaten second to qualify for Monday’s final.