Maria Sharapova will sit out the U.S. Open because of a bad right shoulder, the first major championship she'll miss since her Grand Slam debut in 2003.

The three-time Grand Slam title winner already had announced she's pulling out of the Beijing Olympics because of the injury. Sharapova said in a posting on her Web site Friday she probably won't need surgery and could be ready to play in two to three months.

"It hurts me so much to miss the Olympics and the U.S. Open, you have no idea," she said. "Just to type those words hurt!!"

Earlier Friday, a U.S. Open official told The Associated Press that Sharapova's agent informed the tournament she wouldn't be able to play in the year's last Grand Slam event.

The No. 3-ranked Sharapova has played in each of the past 23 major championships, winning titles at Wimbledon in 2004, the U.S. Open in 2006 and the Australian Open in January.

A doctor who looked at tests on her shoulder from April and this week told Sharapova she has been playing with a torn rotator cuff tendon since the spring.

"He actually couldn't believe that I've been playing this long with this injury. You can imagine that I was not very thrilled to hear that my medical team did not see this tear in my shoulder back in April," she said. "The good news is that it didn't get much worse since April, but we could have started the healing time back then instead of now."

Sharapova will go to Arizona to work with a specialist for rehab and strength work.

"Now I need to move forward and stay positive," she said. "I am going to work hard to get healthy."

The Olympic tennis tournament begins Aug. 11, and the U.S. Open starts Aug. 25.

Sharapova is 32-4 with three titles in 2008, and she briefly was ranked No. 1. The 21-year-old Russian has become one of tennis' biggest stars, with millions of dollars in endorsement deals.

She pulled out of the WTA tournament in Montreal because of the shoulder on Wednesday night — after winning a nearly three-hour match in which she double-faulted 17 times. The right-hander was examined by a trainer midway through the three-set victory.

"After yesterday's match, I knew there was something seriously wrong with my shoulder," Sharapova said in a posting on her Web site Thursday under the headline: "No chance of me competing in Beijing."

Sharapova has been bothered off and on by her right shoulder for some time, forcing her to pull out of a tournament in March.

"Once I'm healthy, I'm sure I'll look at this as a blessing in disguise," she said on the Web site Friday. "Right now it's a bit painful, of course, but every athlete goes through these patches and I'm just grateful that this isn't as bad as it could have been."