Mandela, 94, was hospitalized June 8 for a lung infection and has been in an intensive care unit since then.
Zuma said he visited Mandela Sunday and that he was sleeping, the Los Angeles Times reported.
"I said I am not a doctor. I think what I understand is that when a person is critical, the person is critical. I am not a doctor to describe a condition. To say how critical, that's the business of doctors, so I will stop there," Zuma said.
"I think it's important for us to say that Madiba [Mandela's clan name] is critical in hospital and this is the father of democracy ... and therefore he's a man we all love. I think all of us in the country should accept the fact that Madiba is now old and that therefore as he ages, his health will begin to trouble him," Zuma said. "I think what we need to do as a country is to pray for him to be well and pray for the doctors so that he can be well and come out of the hospital."
Mandela has suffered lung infects repeatedly in recent years; he contracted tuberculosis in the 1980s while serving a 27-years prison term for his work fighting apartheid in South Africa