South African Wayde van Niekerk got his bid for an audacious 200/400m double off to a flying start when he defended his one-lap title on Tuesday.
Van Niekerk, who also set the world record when winning Olympic gold in Rio last year, was in imperious form, running a devastating final bend to finish in 43.98 seconds, easing up a full 15 metres from the line.
Steven Gardiner of the Bahamas claimed silver in 44.41sec, with Sudan-born Qatari Abdalelah Haroun taking bronze (44.48).
"It's an amazing blessing," said Van Niekerk, whom Usain Bolt has pegged as his most likely successor as the athlete capable of dominating the track after the Jamaican sprint star's exit from athletics after these world champs.
"I've got a good team that will help me recover after this and I'll be back tomorrow."
There was drama before the race, however, as the IAAF barred in-form Botswanan Isaac Makwala from competing after he was diagnosed with an infectious disease and ordered to observe a 48-hour quarantine.
That did not stop Makwala trying to get into the stadium before being refused entry.
"The IAAF is very sorry that the hard work and talent of Isaac Makwala won't be on display tonight but we have to think of the welfare of all athletes," world athletics' governing body said in a statement.
And so Van Niekerk now turns to the 200m, in which neither Bolt nor the world's fastest man this season, Makwala, are running.
Should Van Niekerk bag the double, it would be the first since American Michael Johnson achieved the feat at the 1995 worlds in Gothenburg and a year later at the Atlanta Olympics.
© 2017 AFP