Foxes, 25, said she could not believe Williams personally asked for her to be his support act on his tour, reported the Daily Star.
"I couldn't quite believe he knew who I was. He said he was a really big fan, he loved my sound and that I was really unique. I did this cover of his song 'Happy' and we chatted about that. He made me feel really comfortable, he's so chilled.
"I remember being a kid and just looking up to him and thinking he's just the king of music," she said.