American Bob Bryan, one half of the world's top-ranked doubles pair, has pulled out of the Davis Cup semi-final against Spain starting on Friday after aggravating a shoulder injury.

Bryan told the tournament website (www.daviscup.com) that he exacerbated the injury while winning the US Open [ Images ] doubles title with his twin brother Mike.

"I had to shut it down now. I can't even hit any balls," he said. Partner Mike Bryan [ Images ], who has not played a tournament without his twin brother since 2002, said he had been practising with US Open quarter-finalist Mardy Fish [ Images ]. Captain Patrick McEnroe would not confirm if they would be the doubles pair.

"The shoulder has been bothering him since Wimbledon [ Images ]. It's been a long summer, he has been taking anti-inflammatories and it's pretty sore," Mike Bryan told a news conference in the Las Ventas bullring, which still stage the event.

"I'm not a doctor but I think it is pretty serious."

The US team, who will depend on Andy Roddick [ Images ] with 20-year-old Sam Querrey also included, face world number one Rafael Nadal [ Images ] and a strong Spanish team enjoying home advantage on their favourite clay surface.