Bill Cosby has returned to court to fight sexual assault charges which his lawyers say violate an agreement with a former district attorney not to prosecute the disgraced comedian.

Dressed in a suit and tie, the megastar-turned-pariah walked into court in Norristown, just outside Philadelphia, Pennsylvania, using a walking stick and steered by two bodyguards.

It was his first return to court since his lawyers filed a petition last month, asking the court to dismiss the charge that forced the 78-year-old to post bail of $US1 million ($1.42 million) on December 30.

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Cosby’s lawyers say the charge violates a 2005 agreement that Cosby would never be prosecuted over allegations of assault made by Andrea Constand, a former employee of Temple University in Philadelphia.

Ms Constand says Cosby assaulted her at his suburban Philadelphia home in 2004.

Cosby’s wife Camille has refused to give testimony in court. Photo: Getty

The comedian appeared in better spirits than during his bail hearing, when he walked stony-faced through the media scrum.

On Tuesday, he chatted and smiled with his bodyguards and lawyers.

Cosby’s lawyer Monique Pressley says her client is now blind.

Cosby’s team called as a witness the former prosecutor of Montgomery County, Bruce Castor, who reached an agreement that the actor would not be prosecuted over the alleged 2004 assault if he agreed to testify in a civil suit.

Mr Castor told the court there were discrepancies in Ms Constand’s testimony.

If found guilty, Cosby — who has surrendered his passport to the court — could face up to 10 years in prison and a US$25,000 ($35,420 AUD) fine.

More than 50 women have publicly alleged sexual abuse by the pioneering black comedian, who attained his greatest fame for his role as a lovable obstetrician and family man in the hit 1980s sitcom “The Cosby Show.”

But his attorneys repeatedly deny any wrongdoing by the veteran TV actor whose career spanned four decades.

Prosecutors say the charge stems from new evidence in the case that came to light in July, prompting the reopening of the investigation.

Statutes of limitations have prevented most of the women who have accused the comedian of sexual assault from taking legal action.

In the Pennsylvania case, the statute of limitations is set to expire early next year.