Joaquin flipped his car!

LOS ANGELES -- Us Weekly has learned exclusively that actor Joaquin Phoenix was involved in a serious car accident in the Hollywood Hills earlier today. ''His brakes went out and the car flipped over,'' the actor's publicist, Susan Patricola, tells Us. ''But he's doing fine. He may be very sore in the morning, but right now he is doing okay.'' Although the exact circumstances of the accident remain under investigation, his representative says the 31-year-old star was driving the car at the time, but is unknown whether there were other passengers in the car and whether they were injured. The Golden Globe winner was not hospitalized, according to his publicist. Sources tell Us the accident occurred on a winding stretch of Lauren Canyon Boulevard just north of Sunset Boulevard. Phoenix, whose older brother River died at age 23 from a drug overdose in 1993 outside the Viper Room on Sunset Boulevard, has earned critical acclaim for his portrayal of the late Johnny Cash in last year's Walk the Line, costarring Reese Witherspoon. In addition his Golden Globe win on Jan. 16, the actor is nominated for a Screen Actors Guild award, to be handed out on January 29. ''Joaquin will be at the awards on Sunday,'' says his rep. ''He'll be okay.'' [Us Weekly]