"TMZ has learned that R&B star Usher is sick with the flu and has been unable to perform in the Broadway musical "Chicago." Usher, who has been playing Billy Flynn in the production since August 22, missed several shows Friday through Monday. Fans have lined the streets near the Ambassador Theater each night demanding refunds after discovering his understudy would be performing. Word has it that his doctor has ordered him to be on bed rest the remainder of the week (Wednesdays are his night off anyway) and that he may not return to the stage at all. The singer was originally scheduled to perform through October 1, but extended his run until October 14, which is also his birthday. The shows have been sold-out for weeks and Usher is reportedly the highest paid to play the role of "Billy" in "Chicago" of all time. No word on whether he'll make up the sick days and extend his run again.A call to Usher's management company was not returned."