At a hearing in the US District Court in Manhattan on Monday, Andrew Brettler also said the plaintiff, Virginia Giuffre, appeared to have in 2009 signed away her right to sue the Queen's second son in resolving a separate lawsuit.
Giuffre (38) has said Andrew's abuse occurred about two decades ago, around when disgraced financier Jeffrey Epstein was sexually abusing her.
Andrew, the Duke of York, has denied Giuffre's accusations.
"This is a baseless, nonviable, potentially unlawful lawsuit," Brettler, who is based in Los Angeles, said at a court conference conducted by phone.
"There has been a settlement agreement that the plaintiff has entered into in a prior action that releases the Duke and others from any and all potential liability."
Giuffre's lawyer rejected that claim. "We have properly served him," David Boies told US District Judge Lewis Kaplan in Manhattan.
Kaplan directed Boies to propose alternative means to serve Andrew, and rejected Bretter's argument that Hague Convention procedures had to be "exhausted" before applying US procedures. The next conference was scheduled for October 13.
Andrew (61) is a former friend of Epstein, a registered sex offender who killed himself in a Manhattan jail in August 2019 after US prosecutors charged him with sexually exploiting dozens of girls and women.
The prince stepped down from royal duties and saw charities and other organisations distance themselves from him after he gave a BBC interview in November 2019, now widely seen as disastrous concerning his relationship with Epstein.
Giuffre's lawsuit puts Andrew in a tough position because he could be held in default and owe damages if he ignores it, or face years of legal battles by defending himself in court.
According to the August 9 complaint, Andrew forced Giuffre to have unwanted sexual intercourse at the London home of Ghislaine Maxwell, the British socialite and Epstein's longtime associate.
The complaint also said Andrew abused Giuffre at Epstein's mansion on Manhattan's Upper East Side, and on a private island Epstein owned in the US Virgin Islands.
In a September 6 letter, a London-based lawyer for Andrew suggested that Giuffre's lawsuit might be dismissed because she signed a release in 2009 in a separate Florida case covering "claims against persons associated with Jeffrey Epstein."
The lawyer, Gary Bloxsome of Blackfords, said Andrew's lawyers needed to review the release and determine its scope.
"Until we have made that determination, it is difficult for us to give advice as to whether the Duke should voluntarily accept service," he wrote.
Maxwell (59) has pleaded not guilty to charges she aided Epstein's sexual abuses.
She faces a scheduled November 29 trial before US District Judge Alison Nathan in Manhattan.
