Wayde van Niekerk will not defend his world 400m title at this month's World Championships.
The 27-year-old South African, who won the gold medal in Beijing in 2015, returned from more than a year-long absence with a knee injury in February but he has suffered a setback in his recovery.
He will miss the competition in Doha, which runs from September 27 to October 6, as a precaution.
"I'm positive and taking things day by day," said Van Niekerk in a statement. "I am respecting all the calls made by the doctor and respecting my body. For me it is about listening to my body and taking it from there.
"For now, I am not rushing myself or putting pressure on myself. I'm extremely happy and at peace with where I am."