Doubles specialist Bob Bryan will miss out USA’s semi final against Spain this weekend, dealing a major blow to his team.

Bryan, who forms one half of the USA's doubles team alongside twin brother Mike, has aggravated a shoulder injury and will not be fit in time for the clash in Madrid.

The 30-year-old had a cortisone shot two days before the US Open - a tournament he and Mike went on to win - but admitted the rigours of playing for two weeks at Flushing Meadows had contributed to his shoulder getting worse.

"I aggravated it pretty bad at the Open and only made things worse," Bryan said.

"I knew if I wanted a chance to play the rest of this year, I had to shut it down now. I can't even hit any balls."

The Bryans, who are 14-2 since debuting in 2003, helped the US win the Davis Cup last year.

Bob's place is expected to be taken by US Open quarter-finalist Mardy Fish.