Tesla boss Elon Musk is due to take the stand in a Los Angeles court and face the British diving specialist he accused of being a paedophile.
Vern Unsworth was among the team credited with co-ordinating the July 2018 rescue of 12 boys trapped in a flooded cave in Thailand.
Mr Musk, in a now-deleted tweet, described Mr Unsworth as a “pedo guy”.
The entrepreneur gave no evidence to support the comment. He is being sued for defamation.
Lawyers representing Mr Unsworth have described Mr Musk’s tweets as “vile and false”. The British diver is seeking punitive and compensatory damages.
Mr Unsworth’s legal team referred to the explanation as “offensive to the truth”.
As well as agreeing to hear the case, Judge Wilson denied the defence’s request to define Mr Unsworth as a “public figure” - meaning lawyers for Mr Unsworth do not have to prove Mr Musk acted with "actual malice", lowering the bar necessary to win the case.
Jury selection is due to begin on Tuesday at 09:30 local time (17:30 GMT), with the first witnesses - Mr Musk among them - likely to be called later on Tuesday.
The outburst last year appeared to be in response to comments made by Mr Unsworth in an interview on CNN, in which he criticised Mr Musk’s decision to send a purpose-built mini-submarine to the Tham Luang cave in Chiang Rai Province to help with rescue efforts.
Mr Unsworth described it as a “PR stunt”, later adding that Mr Musk could "stick his submarine where it hurts”.
Taking to Twitter, Mr Musk said: "Sorry pedo guy, you really did ask for it."
When questioned about the allegation by other Twitter users, Mr Musk replied with “bet ya a signed dollar it's true”. That tweet was also later deleted.
After Tesla’s stock price dipped by as much as 4%, Mr Musk sent a tweet expressing an apology.
“His actions against me do not justify my actions against him,” he wrote, explaining that his comments were "spoken in anger after Mr Unsworth said several untruths and suggested I engage in a sexual act with the mini-sub, which had been built as an act of kindness and according to specifications from the dive team leader”.
Tesla’s share price recovered.
However, Mr Musk went on to repeat the claim in an email exchange after being contacted by Buzzfeed reporter Ryan Mac.
“Stop defending child rapists,” Mr Musk wrote to the reporter. He had apparently intended the comments to be off the record but did not agree that with Mr Mac prior to emailing his response.
Mr Unsworth is seeking damages for the content of the tweets only, not the email exchange - though Los Angeles District Judge Stephen Wilson said it could be used to illustrate Mr Musk’s state of mind when sending the scrutinised tweets.
Mr Musk’s legal team insisted he would not be seeking an out-of-court settlement. Instead, he will argue that “pedo guy” was not an insult suggesting Mr Unsworth was a paedophile.
“Pedo guy was a common insult used in South Africa when I was growing up,” Mr Musk said in a court filing as part of a failed request to have the case thrown out of court. "It is synonymous with ‘creepy old man’ and is used to insult a person’s appearance and demeanour, not accuse a person of paedophilia.”