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Mr Nash was one of three witnesses to take the stand on Wednesday in the third week of the trial.
He said several times he did not want to be in court - he was summoned to testify through a subpoena.
A close friend of Ms Ventura, Mr Nash said he saw her almost every day while working as her stylist.
He told the jury he witnessed Mr Combs abusing Ms Ventura on several occasions.
Asked what names Mr Combs would call Ms Ventura, Mr Nash said he would refer to her as "slut" and "ho", among other derogatory terms.
The stylist told the court Ms Ventura had confided in him that she did not want to participate in the sex parties.
In one incident, Mr Nash testified that Mr Combs showed up at Ms Ventura's Los Angeles apartment while she was asleep on a sofa.
Mr Combs shouted at her and grabbed her by the hair and started beating her, the court heard.
Mr Nash testified that he and Ms Ventura and an assistant to Mr Combs, named in court only as "Mia", tried to run into a bedroom and close the door, but Mr Combs managed to get in.
The hip-hop mogul continued beating Ms Ventura until she hit her head on a bedframe and was bleeding profusely, the court heard.
Mr Nash said he and "Mia" tried to stop Mr Combs by jumping on his back.
The stylist also said he tried to call emergency services, but Mr Combs told him a member of his security would take Ms Ventura to a plastic surgeon for treatment.
Mr Nash said he saw her on a video call the next day with stitches in her eyebrow.
Having started as an intern with Mr Combs' record label Bad Boy, Mr Nash said he became a stylist when he met Ms Ventura in 2008 and still remains close friends with her.
When a defence attorney asked about the last time they spoke, he said: "Yesterday after she had the baby."
Mr Nash also insisted the defence lawyer refer to Ms Ventura as "Mrs Fine", since she was married to personal trainer Alex Fine in 2019.
Ms Ventura, who testified for four days during the first week of trial, was heavily pregnant while on the stand. Media reports said she had given birth in a New York hospital on Tuesday.
Mr Nash said he also advised the R&B singer what outfits she should wear while testifying.