The 33-year-old singer teamed up with her friend again on her latest album 'No Shame', but she admitted the fact they're ''incredible close'' piled on the pressure in the studio.
Speaking to Australian publication Music Feeds, she said: ''It's actually quite hard working with Mark... I mean, it hasn't really changed, the dynamic, 'cause we're incredibly close.
''He is also like a brother to me. I've known him for nearly 15 years now. He's also best friends with my manager... Actually, it was easier working with him in the early days because I didn't really care what he thought, whereas now, if he says something that's annoying, I really take it to heart.''
The 'Smile' star admitted she has to be careful when she speaks to the 'Uptown Funk' hitmaker in the studio, because there is more than a professional relationship on the line.
Opening up about their friendship, she added: ''It's difficult for me to articulate or to communicate back to him because we have a relationship, so there's a relationship to be saved.
''I can't just be annoyed and storm out and never come back (laughs)... But he's very opinionated and so am I, so sometimes it is a little bit fraught in the studio with me and Mark.''
Lily has even had to put her friend in his place, and suggested sometimes he can get too focused on the work side of things.
She said: ''Also, because he's so successful, I guess he sometimes forgets that I'm Lily and that I knew him before he was a superstar producer.
''I'm sure if he does sessions with younger musicians that he can talk to them in a certain way and get away with it. I'm like, ''Uh-uh! Hey, Mark. I know you. This isn't you.'' So it's an interesting one.''