Manafort's defense has sought to blame Gates for any illegal conduct and cited the embezzlement to impugn Gates' credibility.
Though he was in court to testify against Manafort, Gates offered praise of Manafort's consulting abilities, which he said brought ex-President Viktor Yanukovych "back from the proverbial political dead".
Gates, who has been cooperating with Special Counsel Robert Mueller's investigation, is the star prosecution witness in Manafort's fraud trial in Alexandria, Va.
Gates testified that he and Manafort had 15 foreign accounts they did not report to the US government, and knew that was illegal.
The testimony comes hours after Manafort's defense attorneys accused Gates of embezzling as they sought to paint Gates as an untrustworthy witness.
As well as serving as Manafort's right-hand man in their multimillion-dollar business, Gates was also Manafort's deputy on the Trump campaign.
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Heading into July, Blackburn maintained an early 2-to-1 cash advantage over Bredesen, with $7.3 million in her bank account. Blackburn has billed herself as a "hardcore, card-carrying Tennessee conservative" from the outset of her campaign.
The witness also testified that he had helped the defendant file false tax returns, and had concealed 15 foreign accounts from the USA government. He also admitted to concealing millions of dollars in foreign bank accounts on Manafort's behalf and to falsifying loan applications and other documents to help Manafort obtain more in bank loans.
By Monday, the jury in Manafort's trial had heard from an Federal Bureau of Investigation agent, high-end menswear salesmen, accountants and financial experts, among other witnesses, to testify about the tax and bank fraud that Trump's former campaign chairman had been charged with.
Accountant Cindy LaPorta - who was granted immunity from prosecution - said on Friday she suspected that Gates was misleading her about Mr Manafort's finances.
The jury has heard how Manafort made tens of millions of dollars for work with pro-Russian politicians in Ukraine. "Rick Gates had his hand in the cookie jar and couldn't let his boss find out", he said.
Gates also testified he and Manafort knew it was a crime because they had been notified by Manafort's accountants in emails. With an enormous salary serving his expensive lifestyle (earlier in Manafort's trial, we learned about his taste for ostrich jackets), Manafort had a pretty good racket going on, until he plunged into debt and made the fateful decision to run President Trump's long-shot campaign in 2016.
It's important to note that some of the alleged bank fraud did occur while Manafort was running the Trump campaign.
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Britain has also had a prolonged spell of hot weather with little rain, leaving vegetation in much of the country parched. Faced with wilting troops, the Swiss Army has allowed soldiers to wear shorts and T-shirts instead of standard uniforms.
Throughout his testimony, Gates largely avoided making eye contact with Manafort, who appeared to be watching him.
Gates also said he had filed false expense reports when he worked for Manafort as a way to steal hundreds of thousands of dollars from those accounts.
Instead, prosecutors called a different witness named Paula Liss.
Judge T.S. Ellis has urged - harshly, at times, including on Monday - for the prosecutors to move their case along more quickly. The White House says Mr. Trump feels his former campaign chair is being treated unfairly.
But Laporta's testimony raised the stakes for Manafort. When Robert Mueller began investigating Russian interference past year, Manafort's questionable lobbying work - the sort of activity that has historically rarely been investigated in Washington - was put under a microscope, exposing him and his associates. Laporta said she had no indication that the loan from Deripaska had been paid off. His testimony, given in short, clipped answers as Manafort rarely broke his gaze from the witness stand, follows that of vendors who detailed Manafort's luxurious spending and financial professionals who told jurors how the defendant hid millions of dollars in offshore accounts.
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Cesc Fabregas could be the only player to miss out from that side, after he picked up a minor knee injury in the game. They left the rest so far behind last season that they didn't even have to glance over their shoulders.