The 44-year-old singer met the former 'Watermelon Sugar' hitmaker when he was 21 and found him to be very ''charismatic'', but despite his global fame, she hadn't initially recognised him.
Speaking to Sirius XM's 'Morning Mash Up' show, she said: ''He was just this young, cute, little charismatic person.
''I didn't know at the time who he was.
''He's super flirtatious and I realised who he was and I introduced myself to him and he was like 'Oh, I know'.''
The 'Chandelier' hitmaker previously admitted Harry, 26, had left her feeling ''dizzy'' thanks to his ''smooth moves''.
She said recently: ''That little chap has some moves because he's got swag. He got me when I was like 35 or something, and he was 21.
''I've heard it from a lot of girls. He's got the smooth moves because he's a charming young chappy. But certainly, I felt like I went bright red.''
Sia - who adopted two teenagers last year - recently admitted she finds producer Diplo ''super duper hot'' and offered him casual sex, but insisted she doesn't want a relationship.
She said: ''Much of our relationship is just being spent trying not to have sex, so that we wouldn't ruin our business relationship - because [Diplo is] super duper hot.
''This year I wrote him a text and I said, 'Hey, listen, you're like one of five people that I'm sexually attracted to and now that I've decided to be single for the rest of my life, and I just adopted a son, I don't have time for a relationship. If you're interested in some no-strings sex, then hit me up.' ''
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