The testimony of Ms. Constand, 44, will still be central to the retrial — as it was in the first trial. The defense, which will present its opening statement Tuesday, will almost certainly question why she maintained contact with Mr. Cosby after the encounter and ask why she took a year to come forward to police, both issues raised at the first trial.

In an opening statement of more than an hour, Mr. Steele appeared determined to undercut any credibility issues, mentioning how Ms. Constand only sued Mr. Cosby after prosecutors more than a decade ago declined to bring charges. The current charges were brought after the investigation was reopened in 2015.

The prosecution had some of Mr. Cosby’s prior statements projected onto a screen, including his acknowledgment that on the night of their encounter he had given Ms. Constand three pills — “three friends” he called them — to help her relax without telling her what the pills were, as well as a later admission to her mother that he had digitally penetrated Ms. Constand. Mr. Cosby has said the pills were Benadryl and the sex was consensual.

The start of the proceedings had been delayed for several hours as Judge O’Neill reviewed a motion by Mr. Cosby’s lawyers to dismiss a juror who, they said, had told another prospective juror last week that he thought Mr. Cosby was guilty. Judge O’Neill decided the juror could remain, though he did not disclose his reasoning.

Mr. Cosby’s entry to the courthouse was briefly delayed in the morning by the protest of a topless woman, later identified as Nicolle Rochelle, a former actress who had appeared several times on “The Cosby Show.” She jumped over a crowd barrier outside the courthouse and yelled “Women’s lives matter” before being wrestled to the ground by courthouse deputies about ten yards in front of Mr. Cosby as he walked toward the building’s front doors.