Ramaphosa, 66, took over last year after the African National Congress forced then-president Jacob Zuma to resign after 9 years dominated by corruption allegations and economic problems.
With more than two-fifths of voting districts tallied, the Electoral Commission put the ANC well ahead, with its closest rival the Democratic Alliance (DA) trailing with a distant 25%.
The Economic Freedom Fighters, founded 6 years ago by former ANC youth leader Julius Malema, was nearing 9%.
The final results are expected to be officially certified on Saturday, May 11.
A new projection by the respected South Africa's Council for Scientific and Industrial Research forecast the ANC would win with 57% – a 5 percentage point drop from the last election in 2014.
The party that wins the most seats in parliament selects the country's president, who will be sworn in on May 25.
Ramaphosa has so far faced resistance to his reform agenda, especially from Zuma's allies who still occupy several high-ranking positions in the party and government.
After casting his ballot on Wednesday, May 8, Ramaphosa said the election was "heralding a new dawn...a period of renewal, a period of hope".
"The outcome of this election will be a major boost for...investor confidence."
The ANC's reputation was badly sullied under Zuma. Its support has fallen in every election since 2004 with the party taking 54% in 2016 municipal elections, compared with 62% in 2014's national vote.
Anti-apartheid icon Nelson Mandela and the ANC were swept to power with a landslide in the country's first multi-racial polls that marked the end of white minority rule in 1994.
Most opinion surveys before the vote had suggested the ANC would secure nearly 60% of the vote because of the Ramaphosa appeal and a fractured opposition.
Dirk Coetzee, a professor at UNISA's political science department, said "the higher the percentage for the ANC, the more it will give him (Ramaphosa) bargaining power".
"If Ramaphosa gets below 50% he will be very vulnerable" to challenges from rivals within the ANC, he added.
'The poor are getting poorer'
The ANC has been confronted by deepening public anger over its failure to tackle poverty and inequality in the post-apartheid era.
"We have given them 25 years but the poor are getting poorer and the rich richer," said voter Anmareth Preece, 28, a teacher from Coligny in North West province. "We need a government that governs for the people, not for themselves."
The economy grew just 0.8% in 2018 and unemployment hovers around 27% – soaring to over 50% among young people.
Of the 47 opposition parties in the race, only the main opposition centrist DA and the radical-left EFF are major players.
The DA is hoping to shed its image as a white, middle-class party.
Its first black leader, Mmusi Maimane, is contesting his maiden general election since taking the helm in 2015, and is expected to make modest gains on the DA's 2014 vote share of 22%.
Malema's EFF was predicted to make major gains, growing from 6.3% to a forecast 11%.
"The ANC has taken people for granted. There is some arrogance which has crept in," said voter Mandla Booi, 45, in Port Elizabeth on the south coast.
The EFF, which appeals mainly to young voters and the poor, has campaigned on a policy of seizing land from white owners to give to blacks.
Enforced land redistribution is also ANC policy – alarming some investors.
About 26.8 million voters were registered to cast their ballots but only 65% of those bothered to cast their ballots. – Rappler.com