Pitino testified Wednesday that he met Karen Cunagin Sypher on July 31, 2003, at the restaurant where he'd gone following a golf outing.
Pitino, who's married, said he bought Sypher a drink and the two exchanged small talk after the restaurant closed.
When he got up to leave, Pitino said she whispered something, opened his pants and the two then had sex "very briefly."
"Unfortunate things happen," Pitino said.
Sypher is charged with extortion, asking for cash, cars and a house to stay quiet about the sexual liaison. She has pleaded not guilty, claiming Pitino sexually assaulted her. Police and prosecutors have said her assault claim lacked merit.
A packed federal courtroom watched Pitino's testimony. Sypher, sitting a few feet away at the defense table, appeared unfazed.
Pitino entered the courtroom briskly wearing a dark suit with a white shirt and red tie. He stumbled a bit while taking the witness stand and gave short answers.
Pitino's much-anticipated testimony marked the first time he's talked publicly in detail about his encounter with Sypher. His testimony came after several witnesses said Sypher was flirty and persistent when she approached Pitino.
Restaurant owner Tim Coury testified Wednesday that the two were talking and having a drink at closing time. He told the pair and a Pitino aide to leave through a self-locking door and then went home.
Pitino's driver, Vinnie Tatum, told jurors that Sypher appeared to have had a few drinks and was "flirty" when she joined Pitino's group.
Tatum stayed with his boss when the restaurant emptied, but said he didn't see or hear the two having sex as he sat about 15 feet away behind a brick half-wall. He had left them alone in the bar area because he felt uncomfortable and considered himself "a third party."
Tatum waited at the restaurant until Pitino and Sypher left together in her car. Tatum stressed he had not been drinking and was awake the entire time.
On Tuesday, jurors heard from multiple witnesses who described Sypher's efforts to meet Pitino.
Margaret Cameron, a former professional golfer, testified she was having dinner at Porcini when she ran into Pitino's entourage and began a conversation. Sypher barged into the group, Cameron said, and thrust her cell phone at Pitino asking for the coach to talk to one of her sons.
"She wasn't going to give up," Cameron said. "She wasn't going to stop."
Bartender Melissa Brent testified that Pitino's group were regulars at the restaurant and appeared to be having a good time in a corner of the restaurant when Sypher wearing a white top with a short white skirt attempted to "edge herself into the scene."
Pitino has coached at Louisville since 2001, a job he took after leaving the NBA's Boston Celtics Tim Sypher, served as special assistant to Pitino. Tim Sypher continued to work for Pitino as Louisville's men's basketball equipment manager, but has recently been named director of the Yum Center, the building that houses offices and training facilities for Louisville's men's basketball team.
Karen and Tim Sypher married in 2004. They have a 5-year-old daughter together. The two are in the midst of a divorce.