BRUSSELS (Reuters) - U.S. and Australian Open champion Kim Clijsters will be out of action for at least four weeks due to a painful right shoulder and wrist.

The world number two wrote on her website (www.kimclijsters.be) that she will miss Belgium's Fed Cup semi-final against Spain on April 16-17 and the Madrid Open starting on April 30.

Clijsters said medical staff had advised her to take four to six weeks off to rest and undergo intensive physiotherapy.

"I think this is a great pity," Clijsters wrote. "The Fed Cup is something I was really looking forward to. But I felt that something was out of the ordinary; there were too many minor ailments, one after another, which did not seem to disappear."

Clijsters' pinched shoulder forced her to retire from the Indian Wells WTA tournament in mid-March and she fell in the quarter-finals of the Sony Ericsson Open in Miami last week.

The 27-year-old said she was keen to be back in top form by the time of the Rome Open on May 9 in readiness for the French Open two weeks later. She missed the grand slam tournament with a foot injury last year.

"I really want to be there at this year's Roland Garros... Now there is no other option than to rest and I certainly can't use a tennis racket for the first few weeks," she said.

(Reporting by Philip Blenkinsop, editing by Ed Osmond)