The 39-year-old 'Candy' hitmaker is concerned that the 18-year-old Canadian star is too young to deal with his huge success and fame, reported Sun online.
"There is a list longer than my arm... It's inevitable that someone like Justin Bieber will hit a hard time. You hear about everybody going, 'I'd better go and do something else'. And it gets to you," he said.
Williams had previously said that he is constantly worried about his insecurities too, which will get the better of him and lead him to ruin his life.