US President Barack Obama flies to South Africa Friday hoping to pay homage to the legacy of his critically ill hero Nelson Mandela, who is fighting for his life in hospital.

Mandela's ill health means the two men, who shattered racial boundaries on either side of the Atlantic, are not expected to have a long-anticipated meeting for the cameras.

Still, reflections on Mandela's extraordinary journey from prisoner to president are likely to permeate Obama's three-day stay.

Mandela, who turns 95 next month, was rushed to hospital three weeks ago with a recurrent lung disease and has since appeared close to death.

On the eve of the visit, South Africa's first black president was said to be in a critical condition, but had stabilised since a scare forced his successor Jacob Zuma to cancel a trip to neighbouring Mozambique.

"He is much better today," said Zuma after seeing Mandela on Thursday for the second time in less than 24 hours.

Yet South Africans, including Mandela's family, remain braced for the worst.

"I won't lie. It doesn't look good," daughter Makaziwe Mandela said. But "if we speak to him he responds and tries to open his eyes -- he's still there."

"Anything is imminent, but I want to emphasise again that it is only God who knows when the time to go is," she told local radio.

Meanwhile Obama, the United States's first black president, led a chorus of support for the man he dubbed a "hero for the world."

Mandela's plight has lent a deeply poignant tone to the visit, around which Obama has built a three-nation Africa tour, and his plans could yet be upended by sudden developments in the ex-president's condition.

"The president will be speaking to the legacy of Nelson Mandela and that will be a significant part of our time in South Africa," said deputy national security advisor Ben Rhodes.

"The president will treasure any opportunity he has to celebrate that legacy."

The White House says it is in the hands of the Mandela family and the South African authorities on any aspect of the visit.

"We will obviously be very deferential to the developments that take place and the wishes of the family and the South African government," Rhodes said.

A visit by Obama to Mandela's former jail cell on Robben Island, off Cape Town on Sunday would now take on extra "profundity", he added.

Speaking in Senegal on the first leg of his long-awaited African trip, Obama described Mandela as "a personal hero."

"I think he is a hero for the world, and if and when he passes from this place, one thing I think we all know is that his legacy is one that will linger on throughout the ages."

The US president recalled how Mandela had inspired him to take up political activity, when he campaigned for the anti-apartheid movement as a student in the late 1970s.

South Africans have also been marking the life of a man who led their country out of centuries of white rule.

Outside Mandela's hospital a wall of messages and flowers has become the focal point for a nation saying a long goodbye to one of the greatest figures of the 20th century.

"There is no sadness here. There is celebration. He is a giant," said Nomhlahla Donry, 57, whose husband served time with the revered leader.

Mandela has been hospitalised four times since December, mostly for a stubborn lung infection.

The man once branded a terrorist by the United States and Britain walked free from prison near Cape Town in 1990.

He went on to negotiate an end to white minority rule and won South Africa's first fully democratic elections in 1994.

He forged a path of racial reconciliation during his single term as president, before taking up a new role as a roving elder statesman and leading AIDS campaigner.

He stepped back from public life in 2004 and has not been seen in public since the football World Cup finals in South Africa in 2010.

But Mandela still draws vast global interest, interest which now appears to be wearing on his family.

Mandela's oldest daughter Makaziwe on Thursday slammed the "crass" media frenzy around her critically ill father, likening the press to vultures.

"They violate all boundaries," she said accusing the foreign press of "a racist element" by crossing cultural boundaries.

"It's like truly vultures waiting when a lion has devoured a buffalo, waiting there you know for the last carcasses."

With his health now fading, Mandela's supporters are starting to show signs of resignation -- while preparing to celebrate his legacy.

In the rural village of Qunu where he grew up amid cattle and mud-walled huts, 93-year-old Keqane Keledwane said she was not ready to say goodbye to her neighbour.

"I want him to get well. I don't want to be the only one left behind," said Keledwane, who lives across the road from Mandela's house. "I wish him a long life, my old friend."