After Wayde van Niekerk’s win at the IAAF World Championships Beijing 2015 in 43.48, there was a prevailing feeling that Michael Johnson’s 20-year-old Olympic record, which was just one hundredth shy of that mark, might be in danger in Rio.

Instead, it was his world record* that fell.

Clocking a stunning 43.03, the South African took 0.15 of Johnson’s mark set at the 1999 IAAF World Championships and yet, for much of the race, it didn’t look like Van Niekerk was even guaranteed to win the gold medal.

Kirani James made his usual quick start over the first 200m but Grenada’s 2012 Olympic champion, in lane six, only acted as the pacemaker for his predecessor LaShawn Merriitt in the lane to his inside.

Merritt got up on James’s shoulder as they entered the second bend level with Van Niekerk, but the crucial part of the race came over the 50 metres between 250m and 300m when Van Niekerk, who in hindsight had clearly distributed his effort far, far better, started to gain ground and entered the home straight fractionally in front of his two rivals.

Post-race video analysis showed Van Niekerk passed 300m in 31.0, in comparison to Johnson's 31.66 in Seville, but James and Merritt were also inside the US legend's time.

With 70 metres to go, Van Niekerk was able to maintain his sprint for home and there was the optical illusion of Merritt and James going backwards despite the fact that they were both operating at sub-44-second speed.

South Africa's first athletics gold for 20 years

Van Niekerk continued to stride out and the gap between himself and the other two principle protagonists continued to grow, before getting South Africa’s first Olympic athletics gold since Josia Thugwane won the marathon in 1996.

"I believed I could get the world record," said Van Niekerk. "I've dreamed of this medal since forever. I am blessed.

"These are guys that inspired me: Usain Bolt, Michael Johnson, I learned from them, and even the guys that ran against me today, LaShawn Merritt and Kirani James, these are guys that inspired me."

Behind him, Merritt looked as though he was on his way to repeating the silver medal he earned behind van Niekerk in Beijing last summer but started to tire about 30 metres from home and James just edged past him to reverse their places on the podium in the Chinese capital.

James finished in 43.76, just 0.02 away from his best, while Merritt came home in 43.85, disappointment etched across the US champion’s face.

Almost making it a record four men under 44 seconds in the same race, 2014 world U20 champion Machel Cedenio of Trinidad and Tobago came through strongly over the final 50 metres to get fourth place in a national record of 44.01.

Further back, the revelation of this event in Rio, Botswana’s Karabo Sibanda improved again to 44.25 to become the third best U20 athlete ever over one lap of the rtrack while Bahrain’s Ali Khamis Khamis set a national record of 44.36 in sixth place.

Granada’s Bralon Taplin finished seventh in 44.45 and Great Britain’s Matthew Hudson-Smith was eighth in 44.61.

In addition to Van Niekerk’s world record, and becoming the first man to win an Olympic 400m title from lane eight, the best ever times for place from fourth to eighth were recorded.

Phil Minshull for the IAAF

*Subject to the usual ratification procedures

The seeding of the semi-finals brought the past two Olympic champions together in the first race.

The 2012 gold medallist, Kirani James from Grenada, got the bragging rights over his predecessor, USA’s LaShawn Merritt, although both men were certainly running somewhat within themselves.

James, as has so often been the case, ran a fast first 200m and had a clear advantage coming around the second bend.

Merritt, the 2008 Olympic champion, pushed hard for about 50 metres to let James know he was there before easing up, both men knowing that they were as good as certain of qualifying by right. Merritt let James exert himself a little more to take the win in 44.02, which was eventually the fastest time of the semi-finals.

Merritt took second in 44.21 as Botswana’s Karabo Sibanda had the race of his life and a magnificent final 100 metres to take third in a personal best of 44.47, more than half a second quicker than he had ever run before, his previous best being 45.15.

The second heat saw Trinidad and Tobago’s emerging star Machel Cedenio, running in lane five, push hard over the first 250 metres.

World champion Wayde van Niekerk was two lanes inside Cedenio and got on his shoulder coming off the second bend and briefly edged ahead but Cedenio surged again as the South African eased his foot off the pedal and let the 2014 world U20 champion take the win.

Cedenio clocked 44.39 with Van Niekerek 0.06 further back.

Two-time world indoor champion Pavel Maslak came flying down the home straight to pick up third but had left his effort too late and his time of 45.06 was not enough to ultimately see him through.

The third heat saw the Granadian number two Bralon Taplin do all the hard work from lane six and tow the rest of the field around the Olympic stadium.

Taplin was clearly tying up in the final 50 metres but did enough to hang on to win in 44.44, just 0.06 shy of his personal best, ahead of Great Britain’s Matthew Hudson-Smith in lane eight, who made a huge improvement to take second place in a personal best of 44.48.

The Briton's late surge over the final 50 metres saw him just edge out Bahrain’s Ali Khamis Khamis, who briefly held the lead coming off the final bend, by 0.01 but the latter went through as a non-automatic qualifier after finishing third in a national record of 44.49.

The impression from the semi-finals is that the medal battle in Sunday’s final should still be between the three men who climbed the podium in Beijing last summer – Van Niekerk, Merritt and James – but be prepared for a spate of personal bests for the men behind them.

Phil Minshull for the IAAF

London 2012 Olympic Games champion Kirani James, from Grenada, looked almost majestic when setting the fastest time in the 400m heats, clocking 44.93 despite only really working at full power over the first half of the race.

It will be a warning that James is in great shape although none of the other expected main protagonists had any difficulty in qualifying either, and IAAF World Championships Beijing 2015 gold and silver medallists Wayne van Neikerk and LaShawn Merritt, who both beat James in the Chinese capital 12 months ago in an epic battle which saw three men going under 44 seconds in the same race for the first time, qualifying equally comfortably albeit in slower times.

Trinidad and Tobago’s 2014 world junior champion Machel Cedenio proved to be the fastest performer in the first heat and finished strongly down the home straight to win in 44.98.

USA’s Gil Roberts, second at the US Olympic Trials, also ran to form and took second in 45.27. After not showing much form this season, it was no surprise that Belgium’s former European champion and 2011 world championships was eliminated after only finishing fifth, with three automatic qualifiers from each heat.

Costa Rica’s former world indoor champion Nery Brenes took off like a man possessed from lane two in heat two but Grenada’s Bralon Taplin ran a much more measured race and was able to see that Brenes was closing on him due to the big screens at each end of the stadium.

Off the final bend, Brenes tied up slightly as Taplin held his form and came through to win in 45.15 with Brenes second in 45.53.

World champion Wayde van Niekerek was running out in lane seven in heat three but found himself challenged on his inside by a moderately revitalised London 2012 Olympic Games silver medallist Lugelin Santos, who has been below the radar in 2016 and not even got close to breaking 45 seconds.

Santos back on on the map

Santos was on the shoulder of van Niekerk coming off the last bend before the South African pulled away to win in 45.26 with Santos just outside his season’s best in 45.61.

Lalonde Gordon, the bronze medallist in London four years ago, prevailed in a three-way battle down the home straight to win heat four in 45.24, ahead of Slovenian record holder Luka Janezic who got second in 45.33.

The big surprise was that Great Britain’s two-time European champion Martyn Rooney struggled from the gun and seemed to have little of the zip he showed in Amsterdam last month, crossing the line fifth in 45.60 and he failed to progress.

USA’s 2008 Olympic champion LaShawn Merritt was a very relaxed winner of heat five clearly only working over the first 300 metres despite running blind out in lane eight.

Down the home straight, he glanced to his inside on several occasions to ascertain where his rivals were, and was able to ease off and just stride home at half power in 45.28 with Asian and world U20 champion Abdalelah Haroun of Qatar a distant second in 45.79.

Merritt’s successor in London four years ago, Grenada’s Kirani James employed similar tactics running in lane six of the following heat and was able to almost jog home with 44.93, with Jamaica’s Rusheen McDonald having to work considerably harder to secure second place in 45.22.

Bahrain’s Ali Khamis Khamis was a surprise winner of the seventh and final heat in 45.12, running a well-measured race which saw Czech Republic’s two-time world indoor champion Pavel Maslak scrape into the semi-finals when he finished fifth in 45.54.

Phil Minshull for the IAAF