'X Factor' Singer Sues -- My Gym Was a Hollywood Casting Couch

It's the oldest story in Hollywood -- naive girl comes to La La Land ... only to be seduced by a pervy boss ... so claims a former "X-Factor" contestant in a new lawsuit.

Katie Waissel -- who appeared on the U.K. show -- moved to L.A. last year looking for fame and fortune ... but she needed a job to pay the bills, so she applied at Equinox in Woodland Hills, CA.

Katie claims the manager, Matt Lee, took an instant liking to her and gave her some very personal attention. Katie says Matt interviewed her several times -- at fancy restaurants where they drank. She says it didn't seem odd, because she had "little familiarity with the American hiring process."

She claims Matt then amped it up, showing up at her house with gifts, groceries and taking her out to more lunches and dinners. She says he also insisted she use his Amex card for gas.

Then Katie says things turned ... after a night of drinking she says she rejected Matt's advances, but says she gave in after he guilted her into feeling ungrateful.

She says she complained to Equinox, but they treated her like the bad guy ... so she's suing Lee and the health club for sexual harassment and retaliation.

Equinox had no comment. We left a message for Matt, but got no return call.