The 94-year-old former president who helped free South Africa from white minority rule has had weak lungs ever since he quarried stone on Robben Island during some of his 27 years of imprisonment when he contracted tuberculosis.
A presidential spokesman said there was no significant change in Mr Mandela's condition after he had spent a restful day and was comfortable.
Last week doctors drained Mr Mandela's lung area of fluid which helped him to breathe more freely.
Children wrote loving messages on stones outside his Johannesburg home: "Get well we love you."
Mr Mandela was admitted on Wednesday night to a hospital in Pretoria, the South African capital. It is his third trip to hospital since December.
In December, Mandela spent three weeks in hospital as he was treated for a lung infection and had a procedure to remove gallstones.
A year ago, he was admitted to a Johannesburg hospital for what officials initially described as tests but what turned out to be an abdominal complaint. He was discharged days later.
He had surgery for an enlarged prostate gland in 1985.
Under South Africa's white-minority apartheid regime, Mandela served 27 years in prison, where he contracted tuberculosis, before being released in 1990. He later became the nation's first democratically elected president in 1994 under the banner of the African National Congress, helping to negotiate a relatively peaceful end to apartheid despite fears of much greater bloodshed.
He served one five-year term as president before retiring.
Perceived successes during Mandela's tenure include the introduction of a constitution with robust protections for individual rights and the Truth and Reconciliation Commission, a panel that heard testimony about apartheid-era violations of human rights as a kind of national therapy session.
Mandela last made a public appearance on a major stage when South Africa hosted the 2010 World Cup.
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