Tiger Woods has withdrawn from this week's Kemper Open in Maryland with a back injury to cast doubt on his fitness to compete in the US Open in two weeks' time.

The former Masters champion told tournament officials that his back stiffened up on Sunday night after finishing only 51st in the Memorial Tournament in Ohio and became worse during practice on Monday.

Bev Norwood, a spokesman for Woods with International Management Group, said: "His physical therapist advised him that with the US Open coming up he could aggravate the condition by playing this week and suggested he come to Las Vegas for treatment. This is a precautionary measure to ensure that he is ready and able to play in the US Open."

Kemper Open tournament chairman Ben Bundred said Woods felt his back stiffen on the flight from Ohio to Orlando and the pain worsened when the 22-year-old hit balls in Orlando on Monday morning.

"He thought nothing of it, went to bed, and this morning when he went to work out it hurt worse," said Bundred, who added that the player's therapist Keith Kleven suggested Woods fly to Las Vegas for treatment of what could be a "potential chronic problem".

Woods has played in 39 PGA Tour events since turning professional in 1996 and this is the first time he has withdrawn because of injury. "Tiger has had back pains on and off but his parents have always assumed they were growing pains," said Norwood. "But the therapist he has been working with for six months said there was some irregularities in his lower back."

Woods, whose enormously hard swing clearly puts strain on his back, has already won seven tournaments.