The comments from Xiao Rong contradict months of official statements that 90-year-old Mr Deng is in good health, and appear to be the first stage in preparing China for the likelihood of the patriarch's death.
Xiao Rong, interviewed by the New York Times, said: "His health declines day by day. People have to understand that he's 90 years old, an old man. And some day there will be a day when he passes away. Now he cannot walk . . . He needs two people to support him." But Mr Deng refuses to use a wheelchair. "He feels that after he sits in a wheelchair, he won't be able to get up again. It's the natural order."
As a veteran of the Long March and the architect of China's reform process, Mr Deng remains the most powerful man in China, despite his frailty. In talking of the possibility of her father's imminent death in such a candid way, Xiao Rong's comments are an extraordinary break with the normal obfuscation of Chinese officialdom.
Her interview coincides with a week of intense speculation about China's paramount leader. On Monday, a Japanese report said Mr Deng was in hospital, seriously ill. The foreign ministry responded with a ritual denial that the report was "groundless" and that Mr Deng "enjoys a good health".
On Thursday, a Shanghai newspaper, the Liberation Daily, unexpectedly published the first updated picture of Mr Deng for nearly a year, showing a frail old man, seated in a chair on the eve of last 1 October, National Day.
In the interview, Xiao Rong said her father was not in hospital, but still living at home in the family compound in central Peking. "If there aren't any problems" the family will, as in previous years, go to Shanghai in time for the Chinese new year on 31 January.
Xiao Rong, who for several years has acted as her father's personal assistant, granted the interview before travelling to New York and Paris next week to publicise her biography of Mr Deng. "I know the reason why everyone is so concerned about my father's health - that's very clear," she said.
In theory, the political succession in China has already taken place, with President Jiang Zemin at the so-called "core" of the "third-generation" leadership. In practice, as the Hong Kong and Chinese stock markets demonstrated yesterday by dropping sharply, the world remains convinced that the death of an infirm old man could plunge China into a period of intense political instability.
Mr Jiang is not considered by analysts to have the necessary political support and stature to lead the country in the difficult next stage of economic reform.
With an eye on Mr Deng's record in history, Xiao Rong defended her father's order in June 1989 to turn the troops on peaceful pro-democracy demonstrators in Tiananmen Square.
"If no firm action was taken, China's future would be too terrible to imagine," she said. But she admitted the army had been inexperienced in crowd control. "It was a tragedy. No Chinese person wanted to see something happen like what happened then. Manypeople died both among the ordinary people and among the military, and some of them died very cruel deaths."
Xiao Rong also clarified a historical mystery by detailing the circumstances surrounding the fall of her brother, Deng Pufang, from a window during the Cultural Revolution, which left him paraplegic. She said he was attempting suicide after being beaten by Red Guards. It had been thought previously that he might have been thrown from the window by his persecutors.Reuse content