On paper it is a mismatch, with Mr Weah, a high school dropout and political novice, having to take on a Harvard-trained former World Bank economist and veteran politician.
The former AC Milan and Chelsea player led the field of 22 candidates in Liberia's first election in 14 years, taking 30 per cent of the vote to Ms Johnston-Sirleaf's 19.6 per cent. An iconic figure in his homeland, who has managed and captained the national football team, Mr Weah fought and effective and well-funded campaign that has drawn enormous crowds, many of them former child soldiers, attracted by his messianic style.
The showdown on Thursday between the two presidential contenders will be staged in the capital, Monrovia. The presidential contest will be decided in a 8 November run-off.
Ms Johnston-Sirleaf was the outstanding performer in a previous debate involving all the candidates and analysts had expected her less obviously qualified opponent to avoid a one-to-one debate.
Mr Weah, 39, who played during his football career for AC Milan, AS Monaco, Paris St Germain and Chelsea, and Ms Johnson-Sirleaf have both campaigned on promises to rebuild war-ravaged Liberia's shattered infrastructure and restore basic services such as water and mains electricity.
The presidential and parliamentary polls were the first in Liberia since the war ended in 2003, after the former president and warlord Charles Taylor went into exile in Nigeria. Taylor, whose army included child soldiers high on drugs, is wanted for war crimes by a UN-backed tribunal in Sierra Leone. He is regarded as the culprit for several west African conflicts.
International election observers, diplomats and United Nations officials have praised an election broadly judged so far to have been free and fair. Both of the contenders have said they will ask that a peacekeeping mission remains in Liberia for the foreseeable future.
Liberia, Africa's oldest independent republic, was founded by freed American slaves in 1847 and enjoyed relatively stability for well over a century, becoming a centre for rubber and iron ore production. That prosperity was shattered by a string of conflicts that laid waste to infrastructure to such an extent that even the capital Monrovia has no piped water or state-supplied electricity. Liberia has debts of about $3bn; unemployment is more than 80 per cent. Its national budget has shrunk to $80m from more than $300m before civil war took hold.
"The country is potentially so rich, that once the security and the political situation stabilise, everything is possible. You can double, triple, multiply by ten [the national budget]," Abdoulaye Bio-Tchane, the International Monetary Fund's top official for Africa, told Reuters.