Ellison, who said she previously dated Bankman-Fried, depicted her former boss as an ambitious young man who had no qualms about sharing misleading financial information with lenders and thought he could one day become US president.

Ellison said the hedge fund, Alameda Research, took about $US10 billion ($A16 billion) in FTX customer funds to repay its debts and make investments. The fund gained the money through a $US65 billion ($A101 billion) line of credit it had on the exchange, and from funds FTX customers deposited into an Alameda bank account when FTX lacked its own account.

"He was the one who set up these systems that allowed Alameda to take the money and he was the one who directed us to take customer money to repay our loans," Ellison said on Tuesday.

The testimony by Ellison, 28, was keenly awaited. She is one of three former members of the 31-year-old former billionaire's inner circle who have pleaded guilty to fraud charges and agreed to cooperate with the Manhattan US Attorney's office.

Prosecutors say Bankman-Fried plundered billions in customer funds to prop up Alameda, buy real estate and donate more than $US100 million ($A156 million) to US political campaigns before FTX declared bankruptcy in November 2022 following a collapse that shocked financial markets and left his reputation in tatters.

Bankman-Fried has pleaded not guilty to two counts of fraud and five counts of conspiracy, and has argued that while he made mistakes running FTX, he never intended to steal funds.

In his opening statement last week, defence lawyer Mark Cohen told jurors to question whether cooperating witnesses like Ellison were putting a new, nefarious spin on old decisions by Bankman-Fried which they had originally agreed with.

Gary Wang, FTX's former technology chief, testified that Bankman-Fried falsely tweeted that FTX was "fine" in November as the exchange faced surging demand for withdrawals. A third cooperating witness, former FTX engineering chief Nishad Singh, is also expected to testify at the trial, which could last up to six weeks.

Ellison testified that she met Bankman-Fried while both were working at Jane Street, a Wall Street trading firm. Bankman-Fried left in 2017 to found Alameda, and Ellison followed when he offered her a job as a trader.

"He was very ambitious," Ellison told the court.

She said Bankman-Fried viewed political donations as a relatively inexpensive way to amass power, and even believed there was a 5 per cent chance he could become president himself.

"He said he thought it was very effective that you could get very high returns in terms of influence by spending relatively small amounts of money," she said, adding that he had donated $US10 million ($A16 million) to US President Joe Biden's campaign.

Cohen in his opening statement sought to pin some of the blame for the collapse of FTX and Alameda onto Ellison, saying Bankman-Fried had told her to hedge Alameda's bets but that she did not.

In testimony that could undermine that argument, Ellison said she always consulted Bankman-Fried on big decisions and always deferred to him.

"He was the person I officially reported to, he owns the company, and he was the one who set my compensation and had the ability to fire me if he wanted," she said.

While Bankman-Fried has written blog posts and granted interviews to reporters since his December 2022 arrest, Ellison has maintained a low profile since she pleaded guilty that same month.

In July, the New York Times published a story citing Ellison's private writings from before FTX's collapse in which she described feeling overwhelmed at work and hurt by a breakup with Bankman-Fried.

After defence lawyers acknowledged that Bankman-Fried had shared the writings with a Times reporter, US District Judge Lewis Kaplan revoked his $US250 million ($A390 million) bail and sent him to jail for probable witness-tampering.

Australian Associated Press