South African President Thabo Mbeki will meet Zimbabwe's rival parties in Harare on Monday amid growing doubts over his chances of securing a power-sharing deal to end the political crisis.
Opposition leader Morgan Tsvangirai said on Sunday he would rather quit talks than sign a bad deal and challenged President Robert Mugabe to hold a new election.
Mr Mugabe had threatened to form a government alone if Mr Tsvangirai did not sign a deal last week.
The post-election talks are deadlocked over how to share executive power between Mr Mugabe and Mr Tsvangirai, putting off any chance of rescuing Zimbabwe from its economic collapse.
South African Ministry of Foreign Affairs spokesman Ronnie Mamoepa said Mr Mbeki would meet the leaders of Zimbabwe's negotiating parties in Harare.
Mr Tsvangirai beat Mr Mugabe in an election on 29 March election but fell short of enough votes to avoid a June run-off, which was won by Mr Mugabe unopposed after Mr Tsvangirai pulled out, citing violence and intimidation against his supporters.
Mr Mbeki, mandated by regional countries to mediate in the Zimbabwe talks, has come under repeated fire for not being tough enough with Mr Mugabe.
Other southern African leaders have taken a harder line against Mr Mugabe, but he has refused to budge, and Mr Tsvangirai's Movement for Democratic Change (MDC) has made it clear it has little faith in Mr Mbeki as a mediator.
Mr Tsvangirai told a rally on Sunday marking the party's ninth anniversary that he would not change his position in the power-sharing talks if pressured by Mr Mbeki.
Mr Mugabe's victory in the election run-off was condemned around the world and drew toughened sanctions from Western countries whose support is vital for reviving Zimbabwe's ruined economy.
A smaller, breakaway faction of the MDC, led by Arthur Mutambara, is the third party in negotiations aimed at forming a national unity government.
