Corey Clark, a competitor in the 2003 version of the popular game, admitted that it was wrong to have a relationship with Abdul and be coached by her during the competition.
"Of course," he told ABC News' John Quinones in the "Primetime Live" TV news magazine aired Wednesday night. "That's why we were keeping it a secret."
Meanwhile, the current version of "American Idol" reached its final four on Wednesday, with voters sending home Scott Savol, the shaky singer from Shaker Heights, despite a tongue-in-cheek Internet campaign to keep him.
While seemingly under siege this season, "American Idol" is still a hit with viewers - 23.8 million watched Tuesday night - heading to a May 24-25 finale.
Clark was the central figure in ABC's unusual expose of a network competitor - done during a ratings "sweeps" month. He reached the final 12 contestants in 2003 but was thrown off the show for failing to reveal a past arrest record.
"Primetime Live" showed how Clark serenaded Abdul during an audition, sauntering to the judge's table and kissing her on the hand. Later, he said someone slipped him Abdul's phone numbers.
He called, she sent a car to bring him to her house and they spent the night talking about how to get ahead in the game, he claimed.
"Primetime Live" detailed how Abdul helped Clark get a cell phone and showed pages of phone records it said detailed calls between the 22-year-old contestant and the 42-year-old celebrity - one lasting 155 minutes.
"It felt like she was hitting on me a little bit," he said, "and I liked it."
He described how Abdul came up behind him one night and kissed him on the back of the neck, and that was the night when their affair began.
A representative for Abdul called Clark "an admitted liar and opportunist who engages in unlawful activities." Fox said Clark had never informed the network about his allegations. The network promised to look into them, but noted Clark was writing a book and had an incentive to seek publicity.
ABC also interviewed Clark's parents, who corroborated his story. His mother said she wasn't happy about the relationship.
The network interviewed several former contestants who missed chances to be among the final 12 contestants the year Clark moved forward.
"If these types of things are going on behind the scenes," said Patrick Fortsen, a former contestant "there's really no point to 'American Idol.'"
Clark, who is in the process of recording an album, said Abdul recently contacted him and urged him not to talk about the relationship. ABC played a tape of a cell phone message allegedly left by her.
"I'm just cleaning up my own pathway," he said. "If that involves getting your dirt off my pathway, I'm going to do that."
Remaining competitors in this year's game are long-haired hearthrob Bo Bice of Helena, Alabama; birthday boy Anthony Fedorov of Trevose, Pennsylvania; Vonzell Solomon of Fort Myers, Florida; and Carrie Underwood of Checotah, Oklahoma.
Wednesday's vote was a disappointment for organizers of votefortheworst.com, a Web site that was conspiring to get the least talented contestant the record contract prize. Savol, of Shaker Heights, Ohio, was their pick.
The Web site became a victim of its own popularity when a crush of visitors led to its shutdown Tuesday night.
"We had a big traffic problem yesterday at 5 p.m. (PDT) before the show aired on the East Coast, more than 200,000 hits," founder Dave Della Terza said Wednesday.
The traffic jam infringed on other Web sites on the shared server so the site went down. He was trying to get it back up as quickly as possible, this time on a dedicated server, Della Terza said.
Savol was booted off despite hard-to-please judge Simon Cowell's opinion that his Tuesday performance of "Everytime You Go Away" was his best ever.
By David Bauder