Defending champion Andy Roddick has become the fifth top-10 player to withdraw from next week's Dubai men's championships.

The American, who is playing in the semi-finals of the Memphis championships this weekend, would have to return to the United States after the Dubai tournament for Davis Cup duty against Switzerland.

He joins Rafael Nadal, Roger Federer, Nikolay Davydenko and Fernando Verdasco among the absentees from the $2 million event.

Federer, who will also miss the Davis Cup tie against the United States, is concerned about his back, and Nadal is suffering a knee injury but expects to turn out for Spain in their Davis Cup tie against Serbia.

Tournament sponsors Dubai Duty Free said none of the withdrawals appear to be a result of the recent furore over the refusal to grant Israeli Shahar Peer entry to the United Arab Emirates for the women's championships.

"They have all seemed to have legitimate injuries," Colm McLoughlin, the managing director of Dubai Duty Free, told reporters on Saturday.

"I have no concerns. We have heard that Roddick was on the start of a hernia. I didn't hear any other reports."