The feds’ star witness against Sean “Diddy’’ Combs quietly wept in Manhattan court Tuesday as she revealed excruciating new details about how he beat, sexually abused and humiliated her for years.

Cassie Ventura, 38, riveted jurors with her painful and often beyond-graphic testimony — including about Combs’ baby-oil-fueled, often violent “freak-off’’ sex sessions that could last more than four days.

“Within the first year of our relationship, Sean proposed to me this idea, this sexual encounter he called voyeurism, where he would watch me… [have] sexual activity with another person, specifically another man,’’ the singer testified at Combs’ federal sex-trafficking trial.

“I just remember my stomach falling to my butt,’’ said Ventura, now married to Combs’ former personal trainer and eight months pregnant with their third child.

“I think I was 22 at the time,’’ said the witness, who met Diddy, 55, about three years earlier, when she was modeling in New York City.

“I was confused, nervous, but I also loved him very much, and I wanted to make him happy,’’ Ventura said.

The twisted sex encounters usually involved “the hiring of an escort and setting up this experience so that I could perform for Sean,’’ she said.

“Pretty quickly over time, I knew it wasn’t something I wanted to be doing, especially as regularly as it became,’’ said Ventura, who would soon sign 10-album record deal with his Bad Boy Records label.

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“Eventually, it became a job for me pretty much,’’ she said of the “freak-offs.’’

The witness remained calm and focused through much of her testimony. But her soft voice would go down to a whisper during particularly traumatizing parts — when she would also rest her hands on her belly.

She cried at times, including when looking at a video of the beating Combs gave her in a hotel hallway in Los Angeles in 2016.

Asked at one point whether there was anything she enjoyed about the “freak-offs,’’ Ventura broke down sobbing.

“The time I spent with him,’’ she said.

She also wept looking at a photo of her and her onetime best friend, model Kerry Morgan, who Combs allegedly once threw a wooden hanger at, leaving Morgan with a concussion.

Combs didn’t have any noticeable reaction to his former gal pal’s damning testimony against him, only taking notes and passing them to his lawyers.

When Ventura first entered the courtroom, Combs whipped around in his chair to look at her. She stared straight ahead.

There did not appear to be any eye contact between the pair through the day — as Ventura revealed one detail worse than the next about how he treated her.

Combs, who met Ventura when she was 19 and he was 37, would urinate on Ventura during some “freak-offs,” she testified — and once had an entire blow-up pool filled with bottles of baby oil for the sick sex sessions.

“Sometimes Sean, he himself or the escort, would urinate on me,’’ Ventura said. “Sean urinated at the same time.”

She said that when a male prostitute peed into her mouth during the “humiliating’’ encounters, “It was overwhelming.

“I choked,’’ the witness said.

Combs once also had an inflatable pool filled with bottles of Johnson’s Baby Oil and other lubricants for a hotel-room “freak-off” — insisting the oil be heated and applied “every 5 minutes’’ so his sex-show participants would be “glistening,’’ Ventura testified.

“When we would run out, we would have to call somebody, [Combs’ staff] … or ask the bell desk and give them a tip” to get more, the singer said.

She said the baby oil — stocked in “larger-sized bottles,’’ with about 10 in the pool for her romp with a male escort — “was always heated.’’

She said the lathering-on of the oil “was [Diddy’s] preference. … If he felt you were too dry, he would let you know.

“You need to be shining.”

She described the events as a “very choreographed experience.”

Ventura also described how she couldn’t even fully concentrate on her music career because of what it took to perform up to Combs’ satisfaction for his creepy, voyeuristic events — which occurred everywhere from New York City, Miami, Las Vegas and Atlanta to Ibiza, Spain and Turks and Caicos.

“The freak-offs became a job where there was no space to do anything else but to recover and try to feel normal again,’’ she said.

“Staying up for days on end, taking drugs, other substances. Drinking. Having sex with a stranger.

“Thirty-six, 48, 78 hours. The longest ever was four days, maybe even more, on and off with the breaks. It was significant,’’ she said.

“I was recovering from the drugs, dehydration, just staying awake all that time.”

Combs has pleaded not guilty to the charges, which could land him life behind bars if convicted.

His camp says that while he may have been violent and enjoys kinky sex, he’s no trafficker.

Ventura is set to be cross-examined Wednesday in the case, which a judge said could last through June.