The charges against Manafort are being prosecuted by Justice Department Special Counsel Robert Mueller, who uncovered the alleged wrongdoing during his probe of Russian interference in the 2016 presidential election and suspected collusion with Trump's campaign.
But after initially fighting the charges, Gates pleaded guilty in February and has been cooperating with the special counsel's office since then.
"We're primarily here because of one man - Rick Gates", Zehnle said.
Mr. Gates, who oversaw some of the financial dealings of Mr. Manafort's consulting business, was expected to be a key witness for the government.
Gates is also expected to play a key role in Manafort's second trial, scheduled for September.
The trial, decided by a jury of 6 men and 6 women who were seated after a brief selection process Tuesday, is expected to last several weeks.
Manafort, 69, has pleaded not guilty to 18 counts of bank and tax fraud related to his lobbying activities on behalf of the former Russian-backed government of Ukraine.
White House press secretary Sarah Sanders is expected to address Robert Mueller's Russian Federation investigation and the trial for Paul Manafort, President Donald Trump's onetime campaign chairman, in a Wednesday news briefing. Some documents are necessary to show Manafort's connections with those consultants, prosecutors say. Trump repeatedly has sought to discredit the investigation, which is also looking into any possible unlawful Trump campaign coordination with Moscow.
Despite that, Manafort's case is widely viewed as a test to the legitimacy of Mueller's ongoing probe, which Trump has dismissed as a "witch hunt".
On Tuesday, Mr. Manafort's attorneys accused Mr. Gates of being the mastermind behind the crimes lodged against their client.
Asonye, who described Manafort's "lavish lifestyle" in detail, alleged that Manafort held more than 30 bank accounts that "served no business goal other than to receive and hide income".
But it was clear from the outset that the case would not address these questions: Prosecutors did not once reference Manafort's work for the Trump campaign.
The opening day focused heavily on the defendant's lavish lifestyle, as prosecutors said he lied to put his money ahead of the law.
"I don't anticipate that a government witness will utter the word 'Russia,"' Andres said. "Paul Manafort worked for Ronald Reagan, Bob Dole and many other highly prominent and respected political leaders", Mr. Trump tweeted.
FILE - Paul Manafort arrives for a hearing at US District Court on June 15, 2018 in Washington, DC. That trial, set in the District of Columbia, involves allegations that the longtime political consultant acted as an unregistered foreign agent for Ukrainian interests and made false statements to the United States government.
Mueller's team has indicted or secured guilty pleas from 32 people and three companies, including the indictment of 12 Russian intelligence officials.