Defence lawyers called to the stand a detective who led a 2005 investigation into claims that Mr Cosby drugged and molested a woman at his home in 2004.
The embattled star, 79, earlier told a judge he would not testify to defend himself against those allegations.
Mr Cosby denies the charge and his lawyer has said she agreed to sex.
Andrea Constand, the 44-year-old at the centre of the case, says the alleged incident happened during a visit to his home to seek career advice.