(contactmusic.com) - R+B star Rihanna sympathises with the high-publicised downfalls of troubled singers Britney Spears and Amy Winehouse - but refuses to suffer a similar fate. The Umbrella hitmaker vows to never enter rehab because of a drink or drug problem, insisting she is too tough to collapse under the strains of fame. She says, "I refuse to crack under pressure - in fact I thrive on it. So when I read about Britney and Amy Winehouse who have suffered under it, it's sad. "It is easy to criticise them but it happens. You just have to stay focused and keep your feet on the ground."