Reigning two-time champion Kim Clijsters announced on Friday that she will not be able to defend her titles at the upcoming U.S. Open.

Clijsters is sidelined with an abdominal strain injury.

"Unfortunately, I will not be able to defend my U.S. Open title this year because of a stomach muscle injury. Two weeks of rehab was not enough to heal this injury. Obviously I'm very disappointed. I trained very hard this summer and felt in a good shape to play the U.S. Open."

The former world No. 1 Belgian star beat Russian Vera Zvonareva in last year's finale in New York and topped current No. 1 Caroline Wozniacki of Denmark in the 2009 U.S. Open title match.

Clijsters actually has a 21-match winning streak at the USTA Billie Jean King National Tennis Center, where she hasn't lost since the 2003 U.S. Open final. She skipped the 2004 U.S. Open due to injury, titled there in 2005, and missed the tournament three straight years from 2006-08 because of injury and retirement. The popular Belgian took time off to start a family and came out of an almost-two-year retirement in the summer of 2009.

The 28-year-old Clijsters is a four-time Grand Slam singles champion, including three U.S. Open crowns. She also titled at this year's Australian Open back in January.

Clijsters also skipped The Wimbledon Championships this summer because of injuries and hasn't titled anywhere since capturing her first Aussie Open title seven months ago.

The 2011 U.S. Open will commence August 29 in Flushing Meadows.