Dr Edmund Pribitkin in hospital after the assault. Credit:AP

A teen who viciously bashed a US surgeon and his son during the Australian Open was released on bail months before the January attack, despite a magistrate saying she feared he would kill someone.

"I remember ... I remember," Magistrate Gail Hubble said, when the 17-year-old boy appeared in her courtroom on Friday for the second time in less than a year.

“'I’m worried that you’re going to kill someone', I remember saying that to him."

The boy, who cannot be named for legal reasons, has pleaded guilty to assaulting Dr Edmund Pribitkin and son Edik who were visiting Melbourne for the Australian Open on January 17 this year.