Charlotte Austin, 25, told the Sunday Mirror that she met the 32-year-old superstar while she was working as a barmaid in the Conrad Hotel, in London's posh Chelsea, about 18 months ago.
Williams took refuge in her hotel after being pursued by paparazzi but Austin joked that the photographers were there for her and that he should stop being so self-centred.
"He laughed and he told me I was a fabulous-looking girl," she told the Mirror. "We hit it off right away. I'm not a fan, and I don't like his music at all. And because I took the p... out of him, I think he felt comfortable with me.
"We started chatting every time he came in. He used my name straight away because he could read it from the name-badge on my uniform so I started calling him Rob."
She said Williams would often wander into her hotel alone to sit and drink tea in the evenings and seemed to crave company.
"He went away to LA for a while and when he came back we struck up our friendship properly. He was a real flatterer. He'd always tell me I looked fantastic or fabulous - it was always these really overblown compliments.
"It's a bit empowering - if Robbie Williams tells you you're gorgeous, well it does help to get rid of some of your insecurities. I mean - if he thinks you're beautiful then you have to have something going for you, don't you?"
But it wasn't until March this year that the singer asked for her phone number. "We were just chatting away about the normal stuff - well, my stuff was normal, his was about LA and recording," Charlotte told the paper.
"Then he asked me when we were going to start hanging out. He asked for my number and I gave it to him. I know a lot of girls might have been waiting by the phone, but I wasn't - by this stage I knew him quite well."
Austin said she hadn't heard from the singer for several weeks and was unsure if she would again. She is no longer working at the hotel.