On Monday, Mugabe and Tsvangirai hold what the MDC says will be the last round of talks on outstanding issues from a power-sharing deal such as new appointments of the central bank governor and the attorney-general.
If they are not resolved the MDC’s national council will meet to decide on the party’s next step.
Any crisis in the new government could make it even harder for Tsvangirai and Mugabe to get help from sceptical Western donors, who want to see political and economic reforms before pouring money in to help rescue Zimbabwe’s shattered economy.
"It is only 100 days so far, but this government has consolidated. We have our problems, who doesn’t? Some people are not happy with everything that’s happening," said Tsvangirai.
Morgan Tsvangirai, Zimbabwe’s prime minister, has insisted that President Robert Mugabe is "part of the solution" to the country’s chronic problems as he seeks to win over sceptical donors in the west.
Tsvangirai declared Zimbabwe open for business and eager for investment and described his first 100 days in office, blighted by personal tragedy, as "the most wonderful and awful" of his life.
Critics argue that Mugabe has been winning the battle of wills against his rival since they signed a power-sharing agreement in February. They point to the president’s control of key ministries and the continuing detention of political prisoners and seizures of white-owned farms.
Anxieties that Tsvangirai has conceded too much ground, inadvertently legitimising his rival, were voiced when he attended a public forum with exiled Zimbabweans at South Africa’s University of the Witwatersrand last week.
One member of the audience declared, to applause, that Mugabe "has almost single-handedly destroyed the country, lost two elections and yet is still there … Why do you have to sleep with the enemy? Is not the simple solution that Mugabe goes?"
Tsvangirai responded with a message partly aimed at doubters in the west: "Don’t be too paranoid about your obsession with Robert Mugabe because he isn’t going to go away; he is there. Robert Mugabe was part of the problem but he is also part of the solution, whether you like it or not."
The prime minister said his Movement for Democratic Change, and Mugabe’s Zanu-PF, had resolved nearly all the outstanding issues and that an announcement would be made on Tuesday. It could prove a watershed in what has become a delicate waiting game for the west.
Britain and other countries provided about $670m (£444m) in emergency humanitarian aid to Zimbabwe in 2008 and have pledged more this year, but they have demanded that certain preconditions be met before they will deliver long-term development support.
They were urged to give the unity government the benefit of the doubt last week by the Elders, a group of eminent global leaders including the former United Nations secretary general Kofi Annan, and the former South African president Nelson Mandela.
The Elders said they believed the risks of inaction by donors outweighed the challenges of delivering increased aid.
The group’s chairman, Archbishop Desmond Tutu, said: "The inclusive government needs more support to ensure that it can initiate the urgent stabilisation and early recovery programmes that the people so desperately need. Now is not the time for donors to take a ‘wait and see’ approach. This is the best chance Zimbabweans have had for peace and prosperity in decades."
The Elders said more funds were needed to provide teaching materials, cover school fees, support local food production and rehabilitate the water and sanitation infrastructure, especially after a devastating cholera outbreak.
There is some evidence of increased willingness among investors to gamble on Zimbabwe stabilising. Neighbouring countries and two African banks have pledged $650m in credit lines. The International Monetary Fund said last week that its board had decided to partially lift the suspension of technical assistance.
Patrice Motsepe, a Soweto-born businessman dubbed South Africa’s first black billionaire, visited Harare last month and told Mugabe that, with reassurances, capital would flow in. "I’m very confident and optimistic, Mr President, that two years from now, there will be huge investment in this country," he said.
But the government is still far short of its target of $2bn in emergency funding and $8bn in the long term to help stabilise an economy ravaged by a decade of hyperinflation, unemployment above 90% and political violence.
Tendai Biti, the finance minister, made the case for the lifting of western sanctions during recent visits to Washington and London. He said: "There are some western colleagues who still have bones to chew with us, and we understand that. We welcome African institutions that want to help us."
In an interview with South Africa’s Sunday Times, Tsvangirai meanwhile reflected on "the most wonderful and awful" time of his life. His first 100 days as prime minister were marked by the deaths of his wife and his grandson in separate accidents.
"It has been quite an experience, from the formation of an inclusive government to the trepidation about what’s going to happen, and the personal loss," he said.
"It is only 100 days so far, but this government has consolidated. We have our problems – who doesn’t?.
"Some people are not happy with everything that’s happening, but sceptics are now the minority. The majority believe we are on the right track and I believe so myself."
The newspaper predicted that, in his review in parliament on Tuesday, Tsvangirai would detail the ongoing torture and beatings of political detainees and continuing looting and farm invasions. He would thank the British government for its assistance in paying retention allowances to doctors and nurses so state hospitals could be reopened
"But sceptics are now the minority. The majority believe we are on the right track and I believe so myself."
Eighteen opposition activists arrested on terrorism charges were released on bail on Wednesday. Their indictment and imprisonment brought new tension to the unity government.
Failed talks on Monday could threaten the power-sharing agreement. But Tsvangirai did not seem very concerned.
"I have been disappointed at the slow pace on [the agreement]. But that deadline is set by my party – we have worked through all that. We have viewed [Mugabe’s] approach as a delaying tactic," he said.
The southern African country has said it requires $8.3bn to rebuild an economy ravaged by years of hyper-inflation and contraction.
Although much of the funding is expected to come from sceptical Western donors, Zimbabwean officials are looking to raise about $1bn from the African continent.