Morgan Tsvangirai, the Movement for Democratic Change (MDC) leader, pulled out of the contest on Monday, citing widespread attacks against his supporters as the region for his decision.
He said on Wednesday that he was open to a negotiated settlement with the ruling Zanu-PF, Mugabe's party.
But he suggested that he would only consider talks as long as Mugabe does not declare victory in the second round run-off.
"We are making this proposal, which Mugabe has the choice to accept or not, and I am sure that Zimbabweans want a negotiated way out of the crisis," he told Al Jazeera.
"Both parties [Zanu-PF and the MDC] must realise that this country is burning and the only way is to sit down and find a way out of it."
Leaders of the Southern African Development Community (SADC), said on Wednesday that the election should be delayed while Mugabe's government and the MDC hold talks.
Tomaz Augusto Salomao, chief of the 14-nation SADC, said: "The political situation appears not to be permissive for holding the run-off elections in a manner that will be free and fair."
South Africa said that a senior negotiator was in the Zimbabwean capital Harare discussing a range of option including a postponement of the vote.
On a visit to London, Nelson Mandela, the former South African president, expressed concern over the Zimbabwean crisis and criticised what he said was a "tragic failure of leadership" in Zimbabwe.
"We look back at much human progress, but we sadly note so much failing as well," he said.
The MDC says that at least 80 of its supporters have been killed by Zanu-PF loyalists and another 200,000 people have been displaced in what it calls Mugabe's "campaign of intimidation" to deter people from voting.
Mugabe supporters have denied the allegations.
Hundreds of opposition supporters have taken shelter in the South African embassy in Harare saying that they fear attacks by Zanu-PF loyalists.