The 45-year-old is on life support after his key organs began to fail late on Friday as a result of the effects of motor neuron disease - a condition he was diagnosed with in 2011.
Van der Westhuizen has fought a public battle against the illness, which attacks the body's neurons.
"Joost remains in ICU in a critical but stable condition," the J9 Foundation, the organisation he helped establish to fight the disease, said yesterday.
"He is responding well to his loved ones and putting up an incredible fight. He really is our Superman!"
In his heyday Van der Westhuizen was regarded as the world's greatest scrumhalf.
- Joost fighting for his life in Johannesburg hospitalRugby legend Joost van der Westhuizen is still fighting for his life after being rushed to hospital in a critical condition on Saturday morning‚ his J9 Foundation said in an update on Facebook.
At 1.88m tall and more than 90kg he terrorised defences around the fringes of rucks.
He earned 89 Test caps and scored 38 Test tries, a record that stood until Bryan Habana broke it at the 2011 Rugby World Cup.
Joost captained the Boks 10 times, which included the 1999 World Cup team, when South Africa finished third.
He was also a winner at the 1995 World Cup, which the Springboks famously won 15-12 after extra time against the All Blacks at Ellis Park in Johannesburg.
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