The 94-year-old was admitted just before midnight on 27 March.
A statement from the South African presidency said doctors are attending to the former president, ensuring that he "has the best possible expert medical treatment and comfort".
President Zuma wished him a speedy recovery.
"We appeal to the people of South Africa and the world to pray for our beloved Madiba and his family and to keep them in their thoughts. We have full confidence in the medical team and know that they will do everything possible to ensure recovery," he said.
Mr Mandela spent 18 days in hospital in December undergoing treatment for a lung infection and gallstones.
This is the fourth time he has been admitted to hospital in just over two years.
Mr Mandela contracted tuberculosis in the 1980s while detained on Robben Island where he served 18 of the 27 years he was imprisoned. His lungs are believed to have been damaged when he worked in a prison quarry.
Mr Mandela is widely regarded as the father of the nation for leading the struggle against apartheid.
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