The Australian fighter was at "diabolical risk" of being stripped of his International Boxing Federation title after Mr Lonergan's company sought an injunction stopping the 27-year-old from signing with another promoter.
Under IBF rules, if there was a "legal impediment" preventing a bout taking place Mr Opetaia would be considered unavailable and at risk of losing his cruiserweight belt, court heard.
Gold Coast-based Mr Opetaia had been set to defend his world title for the first time in the coming months.
"If the injunction were to be granted we are at diabolical risk of having a world title stripped from this young man," Mr Opetaia's barrister Franco Corsaro told Brisbane Supreme Court.
However Justice Martin Burns on Friday dismissed the injunction application.
"It's a pretty big round to win," Mr Corsaro said outside court.
"It's important that the IBF know there is no injunction restraining him from signing with anyone else."
NSW-born Mr Opetaia still had 18 months left on his contract with Mr Lonergan's promotional company D and L Events, the court heard.
However Mr Opetaia reportedly told Mr Lonergan he wanted to terminate the deal in February, prompting D and L Events' injunction application.
Mr Lonergan gained prominence when he promoted Jeff Horn's famous 2017 world title upset win over boxing great Manny Pacquiao at Brisbane's Suncorp Stadium.
D and L Events' barrister Tim Matthews told the court on Friday the granting of an interlocutory injunction was "only interim relief".
"Both parties want a trial expeditiously given the agreement only has 18 months to run," he said.
Asked what would happen next, Mr Corsaro said: "If the applicant (Mr Lonergan) wants to pursue then they will keep pursuing it."
Comment has been sought from Mr Lonergan.
Mr Opetaia claimed a gutsy world title win on the Gold Coast in July last year by defeating three-time champion Mairis Briedis of Latvia despite suffering a broken jaw in two places.
"It was pretty stressful but I had to stay positive," Mr Opetaia said outside court of his legal battle.
"This next chapter is going to be huge and I can't wait to kick it off.
"It's the best I have felt the last couple of years."
Mr Opetaia represented Australia at the 2012 Olympics and 2014 Commonwealth Games before turning professional.
He is undefeated in his 22-fight professional career, winning 17 by knockout.
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