In her testimony, which is expected to last through the end of the week, Ms. Ventura was soft-spoken and visibly emotional, dabbing at her nose and eyes with a tissue. She recounted meeting Mr. Combs around 2005, when she was 19.

“He was this larger-than-life entrepreneur, musician,” she said. “Was a fan of the music. I didn’t know too much about him personally.” Soon after, she signed a 10-album deal with his label, Bad Boy. At that time, she said, she and Mr. Combs still had a “platonic” relationship.

“I wanted to be around Sean for the same reasons as everyone else at the time,” she testified. “He was just this exciting, entertaining, fun guy that also happened to have, you know, my career in his hands.”

Their relationship soon became sexual. Ms. Ventura described her first experiences with “freak-offs,” the drug-fueled sexual encounters with male prostitutes that the government contends were coerced. She said those events caused her “nervousness and confusion.” She did not understand how they could be a turn-on, she said, but felt a “responsibility” to please Mr. Combs. “I was confused, nervous, but also loved him very much,” she said.

Later she testified, “Eventually it became a job for me, pretty much.”

Her husband, Alex Fine, was allowed to be present in the courtroom for the beginning of her testimony, but the judge said Mr. Fine would have to leave during a discussion about an allegation that Mr. Combs had raped her in 2018. Mr. Combs’s lawyers argued that they might need to call Mr. Fine as a witness later.