By Simon Evans

MASON, Ohio (Reuters) - Reigning champion Kim Clijsters pulled out of the U.S. Open due to a stomach muscle injury, effectively calling an early end to her season, the Belgian world number three said on Friday.

Clijsters, who also skipped Wimbledon this year with a foot injury, withdrew from her first match back during last week's Toronto Cup with a partial tear of her left stomach muscle.

"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," said the 28-year-old, three-times a U.S. Open winner.

The Belgian had said last week she aims to play in the London Olympics next year but her latest setback throws some question marks over her future plans.

The four-times grand slam winner also said she would have to skip tournaments in Japan and China scheduled for September and October.

"Now there is nothing else I can do than to rest and have treatment every day. If a gradual approach is not taken, I will relapse in the same injury, and therefore I have also no other possibility than to withdrawal from the tournaments in Tokyo and Beijing," she said.

Clijsters has played only five matches over the past five months, during which she has nursed shoulder, wrist, ankle and foot injuries before suffering her latest setback.

She retired in May 2007 before marrying basketball player Brian Lynch and giving birth to a daughter but returned to the WTA tour in 2009, winning that year's U.S. Open just weeks into her comeback.

Russia's Maria Sharapova, who has suffered her fair share of injuries during her career, sympathized with Clijsters.

"It's probably one of the most frustrating feelings for an athlete just sitting there knowing that even before the tournament your name is not going to be in the draw," Sharapova told reporters after advancing to Cincinnati Open semi-finals with a straight sets win over Australian Samantha Stosur.

"It's tough, but I think, Kim, like many of us knows what it's like to deal with tough moments like this."

(Editing by Pritha Sarkar and Frank Pingue)