Rudy Giuliani on Sunday dismissed Michael Cohen's claim that President Trump Donald John TrumpSteele Dossier sub-source was subject of FBI counterintelligence probe Pelosi slams Trump executive order on pre-existing conditions: It 'isn't worth the paper it's signed on' Trump 'no longer angry' at Romney because of Supreme Court stance MORE had advance knowledge of a meeting between his son and a Russian lawyer during the 2016 election, saying Cohen's "default position is to lie."

"If he taped everything else, why the heck didn’t he tape this?" Giuliani said on "Fox News Sunday," adding that he believes Cohen is "capable of doctoring tapes."

Giuliani, who is Trump's lead attorney on the Russia investigation, was responding to a CNN report that Cohen is willing to tell special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE that the president knew that Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE, Jared Kushner Jared Corey KushnerAbraham Accords: New hope for peace in Middle East Tenants in Kushner building file lawsuit alleging dangerous living conditions Trump hosts Israel, UAE, Bahrain for historic signing MORE and others planned to meet with a Russian lawyer who promised damaging information on Hillary Clinton Hillary Diane Rodham ClintonFox News poll: Biden ahead of Trump in Nevada, Pennsylvania and Ohio Trump, Biden court Black business owners in final election sprint The power of incumbency: How Trump is using the Oval Office to win reelection MORE.

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"It’s just flat out untrue," Giuliani said Sunday. "I would’ve been surprised back then, but now that I know all this about [Cohen], it seems to me his default position is to lie."

The former New York City mayor continued to wage an attack on Cohen after it was revealed that the longtime fixer for the Trump Organization secretly recorded at least one conversation with the president.

CNN aired audio last week that featured Cohen and Trump talking about buying the rights to the story of a former Playboy model who claims she had an affair with the president in 2006.

Giuliani, who earlier this year called Cohen an "honorable lawyer," said his opinion has changed following the release of the tape.

"The man is a pathological manipulator, liar. I didn't know that," Giuliani said. "I knew nothing bad about Michael Cohen until all of this started to happen in the last couple weeks."

Giuliani said Trump is "disappointed" but no longer angry over Cohen's secret recordings. The president on Friday accused Cohen of "trying to make up stories."

However, Giuliani argued that the tapes exonerate Trump from criminal wrongdoing, and ultimately damage Cohen's credibility as a witness.

"He’s done so many despicable things his credibility is not an issue," Giuliani said.

Cohen is reportedly under investigation in New York's Southern District for bank fraud and campaign finance violations.