The 51-year-old singer has recalled being ''shocked'' by the incident, which she says occurred when she was just 19 years old, and she managed to defend herself by kneeing David - who passed away aged 67 in November - in the crotch and elbowing him in the face.
She told the Daily Star Sunday newspaper: ''It shocked me. I was a huge fan and loved 'The Partridge Family'.
''I had a big poster of David Cassidy on my wall and I used to kiss him every night before going to bed.
''When he grabbed me and pushed his tongue down my throat, I just kneed him in the b*****ks and would still do that today.''
The assault occurred in a bathroom at London restaurant The White Elephant in May 1985 where David was dining with Sam and her father after she had shot his music video for single 'Romance'.
Recalling the events in her new autobiography, 'Forever', she wrote: ''In the middle of dinner, I got up to go to the toilet, and as I was standing alone in the women's toilets, washing my hands, the door flew open and David Cassidy came storming in.
''Before I really had time to process what was happening, he had pushed me up against the wall and his hands were all over me. I shouted, 'Get off me, David!' in an attempt to stop him. But instead, he just stuck his tongue into my mouth and shoved a hand under my skirt, while the other kept a firm grip on one of my breasts.
''I reacted quickly and instinctively by bringing my knee upwards, striking him right in the balls.
''David jerked back with an 'Ouch!' and grabbed his crotch with both hands. I took a deep breath and shouted, 'Get away from me!'
''David looked up at me, he seemed almost surprised, and then I elbowed him in the face. (sic)''
Sam opted not to say anything to her father when she sat back down at the table because she didn't want him to do a ''stretch in prison'' for beating up the ''first-class creep''.
The 'Touch Me' hitmaker also recalled how she was told to strip from the waist up for the music video shoot and could feel David's member against her.
In the book, she added: ''Whenever he pressed himself against me, I could clearly feel his d**k.
''David seemed to be deliberately drawing out the photoshoot, which took an eternity, and he had an erection the entire time - something he did absolutely nothing to hide.''
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