Jack and Nikki began when Terry Lester was in the role. I'm not 100% but I believe Jack/Nikki really began after Ashley/Victor...which was the last couple of years of Terry Lester's portrayal of Jack. There's some youtube clips of Terry Lester's Jack and Nikki.

Yes PB's Jack and Nikki was the one the melted the screen, Terry Lester was "Jack & Jill" hotness with both BD and Deborah Adair which Terry was the original "Smilin' Jack" playboy! When Jack & Jill was having an the affair old drunk Mrs. C hired a private dick to spy on Jill and got racy pictures and Jill was starting to see John kept getting puzzle pieces until the last revealed it was Jack with Jill and it was Kay 2 sheets to the wind.

Lindsey Wells, an ex of Jack's he was two timing during his first go-round with Diane, snapped the pictures in revenge for him two timing her. She blackmailed Jack & Jill for advancement in the company. Jack tried to fool her into thinking he wanted to reconcile in order to get the negatives and planned a fake wedding ceremony. When she found out it was fake, she then sold the negatives to Katherine, who of course hated Jill. I loved that puzzle storyline!!!! The execution was perfect!! The good ole days (sigh).