Mbulaeni Mulaudzi will not defend his men's 800m title at the World Championships because of injury.
Mulaudzi said a hamstring problem he had been battling this season was now affecting his lower back.
"I need to take this injury seriously so I can be ready for the Olympics next year", Mulaudzi said.
Women's world high jump champion Blanka Vlasic could also miss the event in Daegu, South Korea, because of a hamstring injury.
Mulaudzi, 30, won the 800m title at the 2009 worlds in Berlin.
The South African took silver at the 2004 Olympics in Athens but failed to make the final in Beijing four years later.
Vlasic, 27, will wait for the result of scans before announcing her final decision on Friday.
The Croat has dominated the women's high jump since winning gold at the 2007 World Championships in Osaka.
She repeated the feat at the 2009 event but had to settle for a silver medal at the 2008 Beijing Olympics.