In a hearing which could make or break the media tycoon's hopes of a return to the frontline of politics, judges at the Court of Cassation will decide whether to uphold an appeal court's decision to clear Berlusconi of the most serious charges he has faced in his scandal-stained career.
The AC Milan owner was convicted in June 2013 of having paid-for sex with Ruby when she was 17, a year younger than the legal minimum for a prostitute in Italy.
He was also found to have abused his power to try and cover up what went on at his infamous "bunga bunga" sex parties, resulting in a seven-year prison sentence and a ban from public office.
If his acquittal is upheld, Berlusconi will be free to spearhead opposition to landmark political reforms Prime Minister Matteo Renzi is trying to guide through parliament.
If the ruling goes against him, the fresh appeal trial will have to take place, an outcome which analysts say would dent Berlusconi's authority over his declining Forza Italia party and could be the final curtain in a political career which has seen him serve three terms as Italy's premier.
"For Berlusconi's immediate future, the outcome is quite crucial," said Giovanni Orsina, an academic at the Luiss business school in Rome and an expert on the AC Milan owner's impact on Italian politics.
Orsina believes Berlusconi's legal woes have helped Renzi dominate the political landscape over the last year.
"Berlusconi's clarity of mind basically disappeared (with the tax conviction)," he argued. "Since then he has been uncertain, unclear and politically ineffective.
"Politically, he is in a coma but the moment he draws his final breath is not predictable. Further judicial problems could bring that moment.
"But if the case goes his way – and my feeling is that it is likely to – then he remains a very powerful man. As long as he has the votes in parliament he can damage Renzi."
Renzi relied on support from Berlusconi to steer landmark labour reforms through parliament late last year.
But their alliance collapsed last month after Renzi successfully backed an actively anti-Berlusconi candidate, Sergio Mattarella, to become Italy's new president.
The Forza Italia leader has since vowed to fight intertwined proposals to effectively abolish the Senate and the completion of a new electoral law designed to produce governments with working parliamentary majorities.
For Renzi the two reforms are central to ending decades of weak government which he sees as the root cause of many of Italy's current woes.
But he is battling to keep some of his allies in parliament on board, and a revitalized Berlusconi could present him with major headaches.
Berlusconi is not expected to attend the court in Rome, where the judges could announce a decision by the end of the day.
No evidence of abuse
The pivotal hearing comes just days after Berlusconi, 78, completed almost a year of community service in an old people's home as punishment for a 2013 conviction for corporate tax fraud.
Judges at the first Ruby trial accepted the prosecution's case that Berlusconi had paid for sex with the dancer, whose real name is Karima El-Mahroug, and that he had abused his power to cover it up.
A year later, appeal judges said there was no evidence the billionaire had abused his power or that he knew the Moroccan-born dancer was 17.
Prosecutors had accused Berlusconi of pressuring the police while he was still prime minister to release El-Mahroug from custody when she was arrested for theft – apparently out of concern that she could reveal their liaison.
He insists he only made the call because he thought she was then Egyptian president Hosni Mubarak's niece.
Berlusconi is separately under investigation for allegedly paying off many of the young women who attended his famous soirees in return for false testimony in the Ruby trial.
He is also embroiled in an ongoing trial for allegedly bribing a senator with €3 million ($4.0 million) in 2006 to join his party and destabilize a centre-left government – but that case is set to time out in late 2015.