The leader of the region's economic power has the best chance of brokering a deal between President Robert Mugabe and opposition leader Morgan Tsvangirai to avert further violence in the wake of an election widely condemned as a sham.
Yet African leaders wrapping up a two-day summit in Egypt are growing more critical of Mr. Mbeki's failure to rein in Mr. Mugabe, with some calling for Mbeki to step up his mediation or step aside.
So what's preventing Mbeki from confronting Mugabe?
The cerebral, reclusive leader has never explained his approach. But the Monitor answers five key questions that help show why he sticks to the path he's chosen.
What is Mbeki's relationship to Mugabe?
Mbeki met Mugabe in 1980, when Mugabe had just taken control of the new nation of Zimbabwe from white colonial rulers.
Mbeki was an exiled member of the African National Congress (ANC), and Mugabe set the tone of their relationship at that moment, the older brother instructing the younger brother on how a successful liberation struggle leader should rule.
"When Mbeki was in exile, he was part of the ANC's diplomatic effort, and you worked with and were friendly with African elites who supported your cause," says Steven Friedman, who covered the liberation struggle in the 1980s and '90s, and now is a research fellow at the Institute for Democracy in Southern Africa (IDASA) in Tshwane, as Pretoria, South Africa is now called.
Mbeki's reluctance to criticize other African leaders was noticed early, when Mbeki was the deputy to South African President Nelson Mandela in the late 1990s.
In 1995, Mr. Mandela was highly critical of the Nigerian military government, which was preparing to execute Nigerian poet and activist Ken Saro-Wiwa. Mbeki assured Mandela that Mr. Saro-Wiwa would be spared, but when the military executed him in November 1995, Mr. Friedman says, "Mandela was incensed, he thought he'd been lied to and deceived."
What power does Mbeki actually have to influence Mugabe?
For years, Mbeki's government has argued that pushing Mugabe to the wall only made the Zimbabwean leader more intransigent, and that gentle persuasion worked better. Indeed, with his firm control over all of the levers of power, including a sizeable army, a well-paid police force, and a well-trained intelligence apparatus, Mugabe is not an easy man to push around.
Yet, Mugabe's greatest weakness now is his lack of legitimacy. The June 27 runoff election was conducted outside the bounds of Zimbabwean law, with levels of violence and intimidation condemned by African observers.
Nelson Mandela, and more recently the African National Congress, have broken from Mbeki by publicly condemning the violence surrounding the Zimbabwean elections.
This would give Mbeki cover should he choose to take a harder line with Mugabe.
"Mugabe has all the power, but Morgan Tsvangirai and the [Movement for Democratic Change] have all the legitimacy, and Mugabe needs Morgan more than Morgan needs Mugabe," says Francis Kornegay, a senior researcher at the Center for Policy Studies in Johannesburg, South Africa.
"Everything that has transpired since March 29 [the first round of the elections, which Tsvangirai won] has been illegal," he says. "Mugabe and his clique cannot sustain this much longer. MDC should just wait them out, play hardball."
As for Mbeki's mediation, Mr. Kornegay says, "To mediate in these circumstances does more to shore up Mugabe than anything else."
Do Mugabe's credentials as a 'liberation hero' prevent Mbeki from pressuring him?
African leaders are often reluctant to criticize one another, since they see themselves united in their histories of combating European colonial rule.
Mugabe has a special place among African leaders, because unlike the vast majority of African nations, Zimbabwe won its freedom not because of imperial exhaustion, but with military force.
Yet Mugabe's liberation credentials have been shaken perhaps irreparably, because of the economic collapse of the country and the well-publicized wave of violence Mugabe's supporters have intiated against unarmed opposition activists.
"Initially, when the land invasions began, the reports were so focused on the plight of white farmers, when literally thousands of black people were being beaten up everyday."
This skewed priority of the media taps into a deep well of resentment not just in Zimbabwe, but in other nations that were formally ruled by whites, Friedman says. "I think he'd be mortified if someone stood up and said, 'You are defending white colonial farmers against a black liberation movement.' "
How do South Africa's domestic politics figure in?
Among ordinary South African voters, there is little sympathy for Mugabe and his ruling party. Newspapers portray the brutality of Mugabe's regime. But that sympathy for the Zimbabwean opposition does not translate into sympathy for the estimated 3 million Zimbabwean refugees who now live and work in South Africa, as recent xenophobic attacks in South Africa have shown.
Indeed, the only reliable base of support for Zimbabwe's opposition comes from the Congress of South African Trade Unions (COSATU), which is part of the ruling coalition led by the ANC.
Tsvangirai himself comes from a union background and some analysts say that Mbeki – who has had his own bad relationship with his trade unionist coalition partners – fears that possibility of a trade-union victory next door in Zimbabwe could encourage trade unionists in South Africa to push the ANC toward policies that would undermine South Africa's fragile economy.
"Mbeki's brother has said that Thabo is prejudiced against Tsvangirai," says Friedman, "because Tsvangirai doesn't have a proper college education, while Mbeki got his master's degree in Britain. But to have ... a trade unionist party [like the MDC and South Africa's own COSATU] defeating a liberation movement party [like Mugabe's ZANU-PF or Mbeki's ANC) is not something that Mr. Mbeki wants to encourage in South Africa."
Does Mbeki have concerns about regional security if Mugabe is ousted?
Few analysts see any direct security threat to South Africa if Mugabe's regime crumbled, even if Mugabe's supporters in the military launch a military coup against Tsvangirai.
Certainly more Zimbabwean refugees would flood into South Africa, joining the 3 million Zimbabweans already here.
But the bigger problem would be diplomatic. None of the leaders of the Southern African Development Community, including South Africa, could give formal recognition to the leaders of a military coup.
"This would put Mbeki and the regional leaders in a spot, because they could not recognize a government that has taken over by a military coup," says Kurt Shillinger, a research fellow at the South African Institute for International Affairs in Johannesburg.