Rabuka heads People's Alliance and is a former prime minister who instigated a coup in 1987 before bringing back democratic elections in 1992 and led the country through to 1999.
Bainimarama instigated the 2006 coup and was installed as prime minister the year after before leading FijiFirst to majority government at the 2014 and 2018 democratic elections.
Provisional results four hours after polls closed have People's Alliance's vote in the mid- to low-40s and FijiFirst in the mid-20s.
Counting will continue through the night with provisional results expected by early Thursday morning. But a close race means a final declaration could be delayed until Sunday.
Rabuka expressed doubt Bainimarama would accept defeat.
But he said there was little prospect of a coup, even if Bainimarama decided to contest the result, after Fiji's military commander told his soldiers to respect the outcome of the election and said anything less would be an affront to democracy.
The Pacific nation has been marred by four coups since it gained independence in 1970.
"I accepted my defeat in 1999 and I hope he can do that," Rabuka said.
"We cannot live forever. We cannot rule forever. A successor from an opposition party should be accepted. It is a normal process in democratic systems."
Bainimarama said he would "of course" respect the outcome of the democratic election when he cast his vote but concerns over a possible coup remain in a nation marred by takeovers.
However, some voters remain concerned about potential repercussions if they speak out against the government.
One, who gave his name as Jack, told AAP he wanted a change in government but both frontrunners were coup leaders.
"I want a change in government but when we are given a choice between two thieves, what hope is there for the future?" the 50-year-old said.
People's Alliance party is working with the smaller National Federation Party to expel the incumbent FijiFirst government.
FijiFirst is trying to tie Rabuka to policies he enacted when he took power in 1987 that sought to empower Indigenous Fijians.
Bainimarama says Rabuka hasn't changed and voting for him would endanger the equality of Indo-Fijians, the descendants of indentured Indian workers.
It is a message that resonated with Nemani who told AAP he voted for Bainimarama because he believed all Fijians are equal.
"I'm FijiFirst because he never separated people," he said.
"We are one people. I don't want to go back to before."
Rabuka has apologised for his coup, saying he is a changed man. He is hopeful a coalition with the NFP, lead by Indo-Fijian Biman Prasad, with help build trust with the Indo-Fijian community.
Voter turnout also appears significantly lower than in previous elections with just over 50 per cent of eligible voters having cast their ballot an hour before polls closed.
"It's still, by Fiji's standards, not acceptable," Supervisor of Elections Mohammed Saneem said.
Bainimarama and FijiFirst are running on their record during the COVID-19 pandemic and a strong economy.
Economic growth is set to reach double digits for this year and sit around 8.5 per cent next year, putting it first and third for growth in the region according to the Asian Development Bank.
But Prasad questioned where the economic gains have gone, arguing Fijians are still no better off financially.
William Waqavakatoga from the University of Adelaide said the economic narrative failing to land in line with people's quality of life both during and after the COVID-19 pandemic could motivate them to vote against Bainimarama.
"People question the reality in the claim of several years of economic boom touted by the FijiFirst government," he told AAP.
About one quarter of Fijians live in poverty and this jumps to more than 40 per cent in rural regions.
Cost of living pressures are also biting with inflation eclipsing five per cent and the price of food more than doubling.
This article was made possible through the Melbourne Press Club's Michael Gordon Journalism Fellowship Program.
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