Media playback is unsupported on your device Media caption Cosby was accompanied by Cosby Show co-star Keshia Knight Pulliam on the trial's first day

A witness in the Bill Cosby sex assault trial has told the court he drugged and sexually abused her in 1998.

It is the same method he is said to have used in the alleged 2004 attack on Andrea Constand he is on trial for.

Mr Cosby, 79, denies the charge. His lawyer says she agreed to sex and has changed her story to investigators.

Dozens of women say Mr Cosby assaulted them, but statutes of limitation rules mean he is on trial for only one allegation.

In both incidents Mr Cosby is said to have invited a young woman to his home for career advice before offering her a pill to "relax" her and abusing her once she was incapacitated.

Witness Kelly Johnson said she initially hid the pill under her tongue but Mr Cosby then checked to see if she had swallowed it.

She later woke up partially clothed in Mr Cosby's bed with Mr Cosby behind her, grunting, before he forced her to touch his genitals, she told the court.

"I was trying to say something," she said. "I don't know if I was actually speaking."

Image copyright Christine Cornell Image caption Mr Cosby looked straight down as the alleged victim described her experience

The case is seen as the biggest US celebrity court case since the murder trial of former American football player OJ Simpson in 1995.

It revolved around "trust, betrayal and the inability to consent", prosecutors told the court in Pennsylvania.

Prosecutor Kristen Feden told jurors that the defendant had "used his power and his fame and his previously practised method of placing a young, trusting woman in an incapacitated state so he could sexually pleasure himself".

As Mr Cosby sat a few feet away, the prosecutor urged jurors to look beyond his father role on the hit 1984-1992 television sitcom The Cosby Show.

That wholesome TV image would be "shattered", she said, accusing the entertainer of "heinous crimes".

Defence lawyer Brian McMonagle countered that Mr Cosby was the victim of false accusations.

"Today I get a chance, with your help, to right a wrong," he told jurors. "I get a chance, with your help, to protect a man from the destruction of the rest of his life."

In the courtroom: BBC's Courtney Subramanian

The courthouse, in Norristown, about 20 miles (32km) outside Mr Cosby's home city of Philadelphia, swarmed with reporters as the celebrity arrived, emerging slowly from the back seat of a black SUV.

Inside court he reappeared, dressed in a navy suit and a striped tie, never turning towards the packed courtroom behind him.

He sat stoically, facing forward and periodically leaning over to confer with one of his lawyers. At one point he spoke out to no-one in particular, as one of his associates had walked away, possibly revealing an issue with poor sight. But as members of the jury entered, Mr Cosby - like everyone else in the room - turned his attention to their corner.

Judge Steven O'Neill appealed to the courtroom full of reporters to "let the trial play out", before spending the first hour explaining to jurors their job.

Ms Constand says Mr Cosby drugged and molested her after she visited his home seeking career advice in 2004.

She was 31 at the time and had befriended him through Temple University in Philadelphia, where he served on the board of trustees.

Ms Constand said Mr Cosby gave her three blue pills which made her legs feel "like jelly" and began to grope her.

In 2006, the comedian settled with Ms Constand after providing an undisclosed cash sum to her.

Media playback is unsupported on your device Media caption Cosby accuser Lili Bernard speaks out ahead of trial

Mr Cosby's defence lawyers argued on Monday that his encounter with her was one of many consensual, romantic episodes between them.

His legal team said he had only offered Ms Constand Quaaludes - a sedative widely used recreationally in the US in the 1970s - after she complained of having trouble sleeping.

Mr Cosby's lawyers asked why Ms Constand had returned to his house after she said he had made previous unwanted sexual advances on her.

They also presented evidence that the two had more than 70 phone discussions after the alleged incident, and accused Ms Constand of changing her story to police at least three times.

Image copyright Reuters Image caption Ms Constand says she was drugged and assaulted

Mr Cosby's wife of 53 years was not seen with him as he entered the court in Norristown, near Philadelphia, on Monday.

But Keshia Knight Pulliam, who played his on-screen daughter in The Cosby Show, did accompany him.

Mr Cosby faces three counts of aggravated indecent assault. Each count carries a maximum sentence of 10 years and a fine of up to $25,000 (£19,500).

Media playback is unsupported on your device Media caption "Bill Cosby was like a hawk looking at a little mouse"

The comic - who was at one point the highest-paid actor in the US - has said he will not testify in the trial, which is expected to last about two weeks.

He has speculated that some of the allegations against him could be motivated by racism.

The jury has seven men and five women - two are black and 10 white.

If convicted, Mr Cosby faces up to a decade in prison.