The trial centers on the 2015 birth of the ChatGPT maker as a nonprofit startup primarily funded by Musk before evolving into a capitalistic venture now valued at $852 billion.

Musk, who invested about $38 million in OpenAI from December 2015 through May 2017, gave his account of OpenAI's early years, recounting how he lost confidence that Altman would keep it a nonprofit. Questioned by his lawyer Steven Molo, Musk said by late 2022 he was concerned Altman was trying to "steal the charity."

"It turned out to be true," Musk said on the witness stand, wearing his usual courtroom attire of a black suit and tie.

Altman, OpenAI's CEO, was in attendance at the federal courthouse in Oakland, California, although he was not scheduled to testify on Wednesday. The trial started Monday and is expected to last about four weeks.

Lawyers for OpenAI have rejected the allegations brought in Musk's civil lawsuit and said there were never promises that the company would remain a nonprofit forever. The company has argued Musk's legal challenge is aimed at undercutting OpenAI's rapid growth and bolstering Musk's xAI, which he launched in 2023 as a competitor.

READ MORE: Musk and Altman show up for trial that could reshape AI

During cross-examination, Musk repeatedly pushed back on questions. OpenAI lawyer William Savitt was asking about emails Musk wrote before OpenAI's founding in 2015 on whether it would be better to make it a standard for-profit company and about tax deductions from his donations to the nonprofit.

"Your questions are not simple," Musk said. "They are designed to trick me essentially." Any simple answer, he said, would be misleading the jury.

Judge Yvonne Gonzalez Rogers stepped in, asking Musk to answer whether it's true or false that OpenAI was formed as a nonprofit in December 2015. Musk said in that case, the answer was yes, but added that it is not always simple, comparing it to asking "have you stopped beating your wife?"

"We are not going to go there," the judge replied, to laughs in the courtroom.

READ MORE: Elon Musk sues Apple and OpenAI, alleging they are stifling AI competition

Despite moments of levity, the stakes are high at the trial, which could sway the balance of power in artificial intelligence. Musk's lawsuit seeks Altman's ouster from OpenAI's board. If Musk wins, it could derail OpenAI's plans for an initial public offering of its shares.

Musk's decision to stop funding the company contributed to a bitter falling out between the former allies that's been evident throughout the trial. On Wednesday, Musk said his views on Altman and his OpenAI cofounders had three phases — from initial excitement to losing confidence to a period in late 2022 when he thought "wait a second, these guys are betraying their promise."

Lawyers for OpenAI have said Musk sought to control the company for himself.

Musk repeatedly testified that while he initially sought a majority stake in OpenAI and control of four out of seven board seats, this would eventually be diluted when OpenAI grew and gained more shareholders. He compared it to his stake in Tesla, which he said is now around 15% after he initially had a majority stake when the electric car maker was founded over two decades ago.

OpenAI, however, claims there were no assurances that he would eventually relinquish his board majority.