"I cannot vote for those who tormented me", Mr Mugabe said, hinting he could vote for MDC.
Chamisa is also being supported by a number of former Zanu PF politicians that were expelled from the party for backing Mugabe.
In the country's first election since Mugabe was ousted after 37 years in power, Zimbabwe goes to the polls Monday amid mounting allegations of voter fraud and predictions of a disputed result.
An opinion poll last week saw the MDC close the gap with Zanu-PF from 11 percentage points to three, with 20% of voters undecided.
The President said the few months he has been in power, the new dispensation has opened many opportunities that have reinvigorated the people of Zimbabwe to dream again.
"The more the merrier", Chamisa said in response to a question about Mugabe's endorsement.
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Mr Mugabe also denied that, as president, he had planned to hand the leadership to his wife, Grace, saying it was "utter nonsense", and suggested that ex-defence minister Sydney Sekeramayi should have taken over.
Mr Mugabe addressed the nation for the first time since stepping down in November and declared that "I will not vote for those who have illegally taken power".
"Whoever wins, we wish him well ..." "He seems to be doing well going by his rallies‚" he said. "I was regarded now as an enemy", he claimed.
A run-off vote is scheduled for September 8 if no presidential candidate wins at least 50 percent in the first round.
Little separates the two men's policies but Mr Mnangagwa is 75 and represents the war generation that has ruled since independence in 1980, while Mr Chamisa, a 40-year-old lawyer, embodies the hopes of many young people.
Mr Chamisa has launched blistering attacks on President Mnangagwa and accused the much-criticised Zimbabwe Electoral Commission (ZEC) of trying to fix the election.
They will be voting in presidential, parliamentary and local elections.