The two political newcomers swept aside establishment candidates in the first round, highlighting voters' anguish with the failure of officials to tackle a stagnant economy, high unemployment and poor public services.
Polling stations open at 8:00 am (0700 GMT) for the runoff finale, with 12 hours of voting to decide the North African country's next leader. Exit polls are expected by Sunday evening.
Adding controversy and suspense to the contest, Karoui only walked free on Wednesday -- to a hero's welcome from his supporters -- having spent more than a month behind bars on suspicion of money-laundering.
"This rivalry for Carthage palace (the presidency) is more exciting than a local derby (football match)," said a young man taking part in lively debate on Bourguiba Avenue in central Tunis ahead of the showdown.
On Friday night, Karoui and Saied themselves went head-to-head in a rare television debate, a last bid to woo voters.
Karoui, a 56-year-old business tycoon, appeared relaxed, if at times hesitant. Speaking in Tunisian dialect, he stuck to his key themes of economic liberalisation and fighting poverty.
Serious but also at ease, law academic and independent candidate Saied, 61, called for the decentralisation of power and criticised the North African country's partisan system, in classical Arabic.
The runoff outcome remains uncertain, with a ban on opinion polls, but Karoui received a boost with his newly formed party, Qalb Tounes (Heart of Tunis), coming second in legislative elections only one week before.
Saied topped the first round in the presidential election, held on September 15, with 18.4 percent of votes while Karoui followed with 15.6 percent. Turnout for that round was a modest 45 percent.
Karoui presents himself as a candidate for the poor but spent most of his campaign imprisoned on money-laundering and tax-evasion charges. He was released on his fourth appeal in court after threatening to contest the results.
While the candidates are both seen as anti-establishment figures, the contrast between them is sharp, with Saied nicknamed "Robocop" for his rigid and austere manner.
A social conservative, he has defended the death penalty, criminalisation of homosexuality and a sexual assault law that punishes unmarried couples who engage in public displays of affection.
Saied is an expert on constitutional law who taught at the Tunis faculty of judicial and political sciences from 1999 to 2018, when he retired and went on to launch an unorthodox election campaign that saw him shun mass rallies and instead canvass door-to-door.
The appeal of flamboyant Karoui, who always appears in designer suits, stems largely from his media empire and philanthropic activity.
A former executive for Colgate-Palmolive, in 2002 Karoui launched a media agency with his brother.
After the 2011 uprising that toppled longtime dictator Zine El Abidine Ben Ali, the Nessma TV channel that Karoui founded turned from entertainment programming towards news, becoming one of Tunisia's largest private broadcasters.
Over the past three years, Karoui burnished his reputation with a charity show on Nessma in which he distributed household appliances to needy families.
His arrest in the run-up to the election cemented his status as an outsider -- despite being a longtime key supporter of president Beji Caid Essebsi, whose death on July 25 brought forward the polls.
Karoui, who says the allegations against him are politically motivated, campaigned by proxy through his wife and party.
Although now a free man, Karoui is still under investigation for fraud and banned from travelling abroad.
But if he wins the runoff, Karoui will "receive immunity... and all the legal proceedings against him... will be suspended until the end of his mandate", constitutional law professor Salsabil Klibi told AFP.