The former prime minister who first came to power after he instigated a coup in 1987 said there was a mood for change.
"I think it's going to go very, very well for us," he told reporters after casting his vote on Wednesday morning.
But he noted, "it's always very different to dislodge an incumbent government".
Sitiveni expressed his doubts about Prime Minister Vosaia Voreqe "Frank" Bainimarama accepting the result of the election if he loses.
Bainimarama can go through the Court of Disputed Returns if he's not satisfied with the outcome.
"I'm hoping there will be a flood of votes so going through that course will be futile," Rabuka said.
The former prime minister is confident Bainimarama won't be able to conduct a coup if he loses.
Fiji's military commander told his soldiers to respect the outcome of the election whether they agreed with whoever claimed power or not, saying anything less would be an affront to democracy.
The Pacific nation has been marred by coups, including by Bainimarama who seized power in 2006.
"I accepted my defeat in 1999 and I hope he can do that. We cannot live forever, we cannot rule forever," Rabuka said.
"A successor from an opposition party should be accepted. It is a normal process in democratic systems."
Lines stretched more than 300 metres at some polling stations an hour after voting opened at 7.30am.
Fijians have been given a public holiday and encouraged to cast their ballot. Voting is not compulsory.
The Fijian Elections Office issued a stern warning to essential businesses operating to allow at least two hours for their staff to leave and cast their vote.
Supervisor of Elections Mohammed Saneem said he'd heard reports of businesses threatening to sack workers who didn't show up on time.
"That is not only unjust, it is unconstitutional," he said, referring to Fijians' right to participate in democracy through free and open elections.
Political hopefuls made their last pitch to the public on Sunday before a strict campaign and media blackout was enacted on Monday until polls close at 6pm Wednesday.
Bainimarama took FijiFirst to a democratic election in 2014 and won but his majority was cut to 50.02 per cent at the 2018 poll.
Rabuka's People's Alliance party is working with the smaller National Federation Party to expel the incumbent FijiFirst government.
The election is expected to be close. Preliminary results will be released after polls close, until 7am the next morning, but a final count could take until Sunday.
NFP leader Biman Prasad says it's the best chance the opposition has had since Bainimarama took power to usher in a new government.
He said reassurances by the military commander that the army wouldn't become involved no matter the electoral outcome instilled courage in Fijians.
"They saw there is no threat of a coup and they're flocking to NFP and People's Alliance," he said.
Bainimarama and FijiFirst are running on their record during COVID-19 and a strong economy. He has also spruiked free education, greater access to medical care and better infrastructure under his government.
But Rabuka and opposition parties are arguing Fiji's infrastructure and essential services are dilapidated.
They say poverty and inequality have increased while freedoms have been quashed by a prime minister they brand a dictator.
This article was made possible through the Melbourne Press Club's Michael Gordon Journalism Fellowship Program.