Olympic champion Steven Gardiner of the Bahamas failed to make it into the final of the men's 400m at the World Athletics Championship in Budapest after pulling up injured in his semifinal on Tuesday.
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Gardiner, who missed last year's world champs in Eugene through injury, looked in total control of the third of three semis at the National Athletics Centre, but crashed to the ground with 100 metres to go.
South Africa's world record holder Wayde van Niekerk endured an agonising wait to qualify as one of the two next fastest after missing out on one of the two automatic qualifying spots in his semi.
Van Niekerk, who set the world record of 43.03 sec when winning gold in the Rio Olympics in 2016, clocked 44.65sec in a semi won by Jamaican Antonio Watson.
"I am disappointed," the South African said. "Let's see what I will be able to do another day.
"I have time to recover, regroup and then I will come back to do my best in the final. Hopefully I will able to win a medal."
Van Niekerk expressed sympathy for Gardiner, having himself come back from a terrible ankle injury.
"It is not so nice to see somebody finishing a competition injured like that," he said.
"I know that coming back from the injury is very difficult. I still remember all these dangerous times. I really want to wish him a speedy recovery."
Also advancing to Thursday's final are Britain's Matthew Hudson-Smith, who set a European record of 44.25sec to win his semi, Grenada's 2012 Olympic champion Kirani James, Jamaican Sean Bailey and Norway's Havard Bentdal Ingvaldsen.
The United States will be represented by Vernon Norwood, a multiple relay medallist, and Quincy Hall.
"I came here to Budapest aiming to win the gold. Now I am one step closer. I am confident and I feel the strength," said Hall.
World indoor champion Jereem Richards of Trinidad and Tobago missed out after finishing behind Van Niekerk.