Officers from the U.S. Federal Bureau of Investigation (FBI) arrested Michigan residents Scott Sippola and Allison Coss on an extortion charge on 2 December (09) after they allegedly threatened to release snaps of the ER star unless he handed over $680,000 (£453,000).
On Monday (12Jul10) it was claimed in Michigan's U.S. District Court that Stamos romanced Coss when she was a high school student in 2004.
In the opening statement, defence attorney Sarah Henderson detailed how Stamos met Coss when she was on vacation in Florida shortly after he had separated from his wife Rebecca Romijn.
Henderson stated that Stamos and Coss kissed and he allegedly offered to perform a sex act on the teenager, but she refused, reports the Associated Press.
Stamos declined to comment following the claims, but his attorney, William Sobel, said in a statement outside court, "The allegations made today in the courtroom by the defendants' attorneys during opening statements will not be proven because they are simply untrue."
Lawyers for both Coss and Sippola are calling for Stamos' lifestyle to be scrutinised in court because they believe it would influence his "credibility".
Federal prosecutors insist Sippola and Coss do not possess photos of the star, and that his private life should not be discussed during the trial.
The case is due to resume in court on Tuesday (13Jul10).