After Microsoft invested $10 billion in OpenAI in early 2023, Elon Musk said in a social media post that OpenAI was “effectively controlled” by the tech giant.

Shivon Zilis, a confidante of Mr. Musk who had spent years working for his companies and had approved the Microsoft deal as a member of OpenAI’s board of directors, disagreed.

“You are naïve,” he told her.

Ms. Zilis, who is also the mother of four of Mr. Musk’s children, described her private conversation with the world’s richest man during testimony on Wednesday in a blockbuster trial in an Oakland, Calif., federal court that pits Mr. Musk against OpenAI and its chief executive, Sam Altman.

Mr. Musk sued OpenAI two years ago, accusing the artificial intelligence company of breaching its founding contract by putting commercial gain over the public good. He founded OpenAI as a nonprofit in 2015 with Mr. Altman and a group of A.I. researchers. But after a power struggle with Mr. Altman, Mr. Musk quit.

Mr. Altman and the other founders later attached a for-profit company to the A.I. lab and began raising billions of dollars from Microsoft and other investors. Now Mr. Musk is asking for $150 billion in damages and a court order that would unravel the for-profit company that OpenAI created. He also wants to remove Mr. Altman from OpenAI’s board of directors.

Ms. Zilis had a unique view of the yearslong struggle between Mr. Musk and Mr. Altman. She started working for OpenAI as an adviser in 2016; developed friendships with Mr. Altman and OpenAI’s president, Greg Brockman; and had a romantic relationship with Mr. Musk around the same time, according to her testimony.