Tsvangirai, leader of the main opposition faction, had previously refused to sign even a framework deal unless government militias stop violence he says has killed 120 of his supporters. He also demanded that Mugabe recognise his victory in the first round of the presidential poll on March 29.
The MDC leader pulled out of the run-off because of the heavy violence between the two rounds.
Mugabe, 84, blames the opposition for the bloodshed.
MEDIATION BROADENED
The turning point in ending the deadlock appeared to be a meeting last Friday between Mbeki, the African Union's top permanent official, Jean Ping, and U.N. envoy Haile Menkerios.
Mbeki, who has up to now negotiated alone as the designated regional mediator, agreed to expand the mediation process to include the African Union, United Nations and officials from the Southern African Development Community in a "reference group".
Mbeki is expected to liaise with the group although it will not be directly involved in negotiations.
Expansion of the mediation beyond Mbeki has been a key demand of Tsvangirai, who has strongly criticised the South African president, accusing him of favouring Mugabe.
Western powers, who have unsuccessfully tried to push targeted sanctions against Mugabe's circle and an arms embargo through the United Nations, also called for expanded mediation.
Mbeki, who favours a softly softly approach to Mugabe, has failed to end the crisis since he began mediating last year and has come under criticism both at home and abroad.
"Tsvangirai has so far done well to press for and win a role for both the AU and the UN in the mediation process, but he has to remain alert all the way because he is dealing with a foxy man - Mugabe," said Eldred Masunungure, a political science professor at the University of Zimbabwe.
"The actual negotiations are going to be a lot tougher and the MDC's aim of easing Mugabe out of power or sharing executive power (with ZANU-PF) in a transitional government ahead of another election is going to be more difficult to get," he said.
