Under questioning by prosecutors, Stamos said he met Coss in Orlando, Fla., in 2004 when he was taken there by friends trying to cheer him up after separating from his wife, actress and supermodel Rebecca Romijn.
"I was very heartbroken at the time," he said.
Stamos said he met Coss at Pleasure Island, a club where patrons must be at least 18. The 46-year-old Stamos said Coss told him she was on "college" spring break.
Stamos said Coss and a friend were among several people who came to his hotel room and stayed for a couple hours. He described their time together as "just hanging out" and "socializing."
He said he later began communicating with Coss via e-mail and described their conversations and "flirty" but "friendly."
"It was all very sweet. I considered her a friend," Stamos said.
Defense attorney Sarah Henderson had alleged that Coss met Stamos when she was 17 and that the two had a romantic fling, which his attorney denies.
Henderson said during opening statements on Monday that two women who worked as strippers eventually showed up in Stamos' hotel room with a bag of cocaine, and Coss and her friend took a picture of Stamos bending over a table where the drugs had been laid out.
Henderson said Stamos and Coss later kissed on a bed and got into a hot tub together after Stamos undressed and Coss stripped to her underwear. She said Stamos offered to perform oral sex on Coss, but she declined. Florida law makes it a second-degree felony for someone 24 or older to have oral sex with anyone 16 or 17 years old.
Stamos eventually became frustrated, broke a bedpost with his hand and left the room before apologizing and inviting Coss to spend the night, which she did, Henderson said.
Stamos declined to comment to The Associated Press after Monday's opening statements. But his attorney, William Sobel, issued a statement through publicist Matt Polk denying the defense's claims.
Prosecutors also asked Stamos to read from a large book of e-mails in court. Among them, Stamos testified, were two e-mails he received last September from a "J Taylor," who claimed she was pregnant and Stamos was the father. The second e-mail voiced concern that he had not replied and read, "If this gets out, I am screwed. But most of all you are screwed 10 times more than me."
Afterward, Stamos testified, he received e-mails from Coss in which she tells him she has cervical cancer and is worried about photos a friend has. In the e-mail that Stamos read, Coss tells Stamos that she is going to meet with a man who has these pictures and has been harassing her.
In the e-mails read in court, Coss appears to become more worried about the pictures while Stamos maintains there is nothing in them to be concerned about. In one e-mail, Stamos asks how he can contact the man and writes, "I'll bust him up. ... We didn't take any bad pics. I'm too smart."
Stamos testified that he also started exchanging e-mails in November with the man known as "Brian L," who says the photos contain images of drug-taking, alcohol use and strippers. The man tells Stamos that he has been offered $780,000 from a tabloid and offers to sell them to Stamos for $680,000, according to the e-mail read by Stamos in court.
Stamos said around this time he contacted the FBI. When asked why by a prosecutor, Stamos said, "I felt threatened, violated. I felt this was illegal."
He said the FBI worked with him and eventually took over his e-mail account and handled all the correspondence.
Two agents testified Monday about a sting operation that ended with Coss and Sippola's arrest at K.I. Sawyer International Airport near Marquette, in Michigan's Upper Peninsula, where an agent posing as a Stamos representative had promised to leave a bag of cash.
Stamos did not say who the "J Taylor" and "Brian L" e-mails were from, but prosecutors have said Coss and Sippola sent e-mails using pseudonyms.
Henderson acknowledged Coss sent e-mails but described them as a ruse to test Stamos' reaction in hopes of determining whether he was preying on young girls.
