In addition to his 2016 Olympic bronze medal and the 2018 Commonwealth title, Bird-Smith had finished 10th, 8th and 6th in the 20km event at the previous three world championships.

But he found the going much tougher in Doha. Shortly after crossing the finish line in 1:32:11, an exhausted Bird-Smith fell forwards onto the road.

The 27-year-old was taken immediately to the medical tent for observation before being cleared to return to the team hotel.

Fellow Australian Rhydian Cowley withdrew midway through the race.

All of the walks and both marathons in Doha have started near midnight to avoid the worst of the heat.

But the conditions have still proven brutal, with the men's 20km event beginning with the temperature sitting at 32 degrees Celsius along with 70 per cent humidity.

Yamanishi's winning time was a full three minutes adrift of the previous slowest, recorded by a 20km gold medallist at a world titles.

AAP