American Idol season 1 runner-up Justin Guarini is reportedly spilling Idol romance secrets. The singer, who lost first place of the show to Kelly Clarkson, recently revealed his past Idol hook-ups.

Guarini recently performed his one-man show at the Bucks County Playhouse in New Hope, which is in Lehigh Valley of Pennsylvania. In the show, titled Lovesick, Guarini reportedly revealed that he had an intimate relationship with Kelly Clarkson.

"He actually had a serious thing with Kelly Clarkson, who won the first 'Idol' crown over him in 2002," a Lovesick reviewer wrote. "It started on the set of From Justin to Kelly, the quickie beach movie they did together after 'Idol,' and ended when her career really took off and his didn't."

Clarkson is now a household name after winning the first season of American Idol. Back in 2012, however, Guarini said the star was "like my sister."

"Yeah, there was a rumor, definitely," Guarini told Yahoo. "We've just always been such good friends. Um, man, it was weird; she was like my sister on the show, and then you know, we made a movie together ... I'd say, we had a good time. Yeah."

According to Radar Online, Guarini dedicated an emotional rendition of Brenda Russell's "Get Here" to Clarkson. Critics reportedly said the performance showed "heartbreak" over the end of his and Clarkson's relationship, and it was the "highlight of the show."

In addition, Guarini's performance also allegedly dished that he had a thing with Tamyra Gray. Gray was fourth place in season 1 of American Idol. She has since appeared in Boston Public and on Broadway. According to Lehigh Valley Music, Guarini "romanced" Gray on the set of Idol.

Guarini can be seen on Broadway in Wicked starting Feb. 25. He has been married for four years to his wife, Reina, and has two children.