Ventura, a rhythm and blues singer known as Cassie, is the star government witness against Combs, who has pleaded not guilty to five felony counts of racketeering conspiracy, sex trafficking and transportation to engage in prostitution.
If convicted on all counts, the 55-year-old rapper and founder of Bad Boy Records would face a minimum 15 years in prison and could face life behind bars.
Combs is being tried in Manhattan federal court.
He is being held in a Brooklyn jail when not in court.
Ventura, pregnant with her third child, spent one-and-a-half days answering questions under oath from a prosecutor, and a lawyer for Combs said the cross-examination may take two days.
In testimony on Wednesday, Ventura told jurors how Combs coerced and blackmailed her throughout their 11-year relationship, which ended in 2018, into days-long, drug-fueled sexual performances he called "Freak Offs".
The trial has drawn intense media coverage because of Combs' fame and towering cultural influence before a wave of abuse allegations led to his arrest and indictment last September.
Part of the criminal case stemmed from Ventura's November 2023 civil lawsuit against Combs.
She testified that he agreed after 24 hours to settle for $US20 million ($A31 million).
Lawyers for Combs have signalled they will ask Ventura about what they have called her own history of domestic violence and have suggested she was motivated by money to get back at him.
Asked on Wednesday why she decided to testify against Combs, Ventura said she could no longer bear the emotional burden of years of his physical and emotional abuse.
"I can't carry this any more," Ventura said.
"I can't carry the shame, the guilt, the way he treated people like they were disposable. What's right is right, what's wrong is wrong. I came here to do the right thing."
Also known during his career as Puff Daddy and P Diddy, Combs founded Bad Boy Records and is credited with helping turn artists such as Mary J Blige, Faith Evans, Notorious B.I.G. and Usher into stars in the 1990s and 2000s.
Combs also faces dozens of civil lawsuits by women and men who accuse him of sexual abuse.
He has denied wrongdoing and said his relationships were consensual.
The criminal trial could take up to two months.
1800 RESPECT (1800 737 732)
National Sexual Abuse and Redress Support Service 1800 211 028
