"Why should it (retirement) be discussed when it is not due?" he said in an interview broadcast on state television.
"The leadership still exists that runs the country. In other words I am still there ... When the day comes and I retire ... I do not want to leave my party in tatters. I want to leave it intact."
Mugabe claimed he is "fit as a fiddle," but appeared frail in the prerecorded televised interview, at times stumbling over his words and slumping in his chair.
Mugabe returned from Singapore today and will celebrate his birthday at a sports stadium tomorrow.
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Tomorrow's birthday celebrations, estimated to cost USD 1 million, will be held in a 50,000-seat stadium in Marondera, 74 kilometres east of Harare, where organisers said potholed streets have been repaired for the event.
Critics say Mugabe won't discuss his retirement because he wants to die in office.
"The truth is we are faced with a very sick president who doesn't want to retire," analyst Ibbo Mandaza told The Associated Press.
Vying to replace him are Vice President Joice Mujuru and Justice Minister Emmerson Mnangagwa.
In July, Mugabe who has ruled the nation for 33 years since 1980, won disputed elections for another five-year term that will take him to age 94.