Jonnetta Patton, Usher's manager, said in a statement, "Usher is currently being treated for an extreme case of strep throat. He fell ill last Friday and his condition became progressively worse, so under strict orders from his doctors, Usher has been instructed not to perform. He is really saddened that he cannot close out his last week in Chicago with a bang. Usher has had an amazing time on Broadway and loved every minute of performing on stage with the entire cast of Chicago. He expresses his sincere apologies to all of his fans who purchased tickets to see him in Chicago this final week and appreciate all of their love and support. He is thankful to the producers, the entire cast and production crew of Chicago."

Usher had been expected to return to the musical Oct. 9. At that time, a spokesperson for the musical told Playbill.com, "We hope that he will return tonight [Oct. 9], as he is anxious to return." His illness prevented that return.

Usher had recently extended his stay with the Kander and Ebb musical through Oct. 14. Christopher McDonald will succeed Usher as the slick lawyer beginning Oct. 15. Huey Lewis begins his second stint as Flynn Nov. 20. (Understudy Bernard Dotson is currently playing the role.)

In a separate statement, Chicago producer Barry Weissler said, "The entire Chicago company is sending their thoughts and best wishes to Usher for a quick and speedy recovery. Usher made a spectacular Broadway debut, bringing a great dedication, work ethic and his amazing talent to the show. We all hope that he might return at some point for the many fans that were unable to see his wonderful performance as Billy Flynn. Usher has found a new home on Broadway and is welcomed back anytime."

The Broadway production of Chicago also features Brenda Braxton as Velma Kelly, Bianca Marroquin as Roxie Hart, Rob Bartlett as Amos Hart, Roz Ryan as Matron "Mama" Morton and R. Lowe as Mary Sunshine. Usher is a five-time Grammy Award-winning artist. His fifth album, "Confessions," was released in 2004 and sold over 15 million copies. The Hip-hop artist has also appeared in such films as "She's All That," "The Faculty," "Light it Up," "Geppetto," "Texas Rangers" and "In the Mix." His television credits include "The Twilight Zone," "7th Heaven," "Moesha" and "American Dreams." Chicago marks Usher's Broadway debut.

Chicago won the Tony Award for Best Revival of a Musical in 1997 as well as awards for actors Bebe Neuwirth and James Naughton, director Walter Bobbie, lighting designer Ken Billington and choreographer Ann Reinking. The original production was directed and choreographed by the late Bob Fosse.

Chicago plays the Ambassador Theatre, located at 215 West 49th Street.