NYPD officer James Frascatore, seen in a video throwing former tennis star James Blake to the ground and arresting him outside a Manhattan hotel, testified yesterday he took such strong physical action because he mistakenly believed the athlete was a fraud suspect who might have been armed with a knife.

In testimony at his departmental trial, Frascatore, 40, of Nassau County, said he ran across 42nd Street to arrest Blake on Sept. 9, 2015 outside the Hyatt Regency to get the element of surprise and had no intention of hurting him.

“I was told he was carrying a knife — that was good enough for me,” Frascatore said, explaining the physical way he took Blake down to the sidewalk and handcuffed him.

But Frascatore admitted he and fellow cops quickly determined they had wrongfully targeted Blake, who was waiting for a ride to the U.S. Open, and saw that he wasn’t the suspect they were seeking in an identity fraud investigation. Blake was then allowed to go, officials said.

Frascatore, a six-year NYPD veteran who is on modified assignment, is facing a departmental charge that he used improper physical force against Blake. He is being tried by a Civilian Complaint Review Board prosecutor before NYPD deputy commissioner for trials Rosemarie Maldonado.

Defense attorney Stephen Worth said Frascatore could potentially face termination if found guilty or perhaps just lose some vacation days.

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Blake testified yesterday that he first thought Frascatore might be an old friend running up to him as a joke outside the hotel.

“I thought it was a friendly encounter until I was on the ground,” Blake, 37, testified under questioning by Jonathan Fogel, the CCRB prosecutor.

Blake had been prepared to shake off the encounter but after speaking to his wife Emily, decided to publicize the police action as a way of holding cops accountable.

“This should never happen to other people,” Blake said in his testimony. “There has to be accountability to everyone.”

“I don’t think we would be here if we didn’t have the video,” Blake told reporters outside police headquarters. “If not for that it would be my word, as credible as I am, against five officers who would most likely tell a different story.”

Blake said that while he initially intended to sue the city, he withdrew a notice of claim and ultimately settled after the city agreed to fund a fellowship at the Civilian Complaint Review Board and agreed to pay him and his attorney $171,000 in expenses.

Worth told reporters the video showed Frascatore acted professionally in taking Blake down.

“Police, despite all their best efforts, make mistakes; it is a human endeavor and human endeavors are imperfect,” said Worth.

The trial is scheduled to resume today with testimony by an expert on police tactics.