Rafael Nadal warned the decision of the Spanish Tennis Federation to host their Davis Cup semi-final against the United States in Madrid could backfire.

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• Spain overrule rebel players

Nadal and the rest of the Spanish players, furious that their wish to play the tie at sea level was ignored, signed a letter saying they would refuse to appear at promotional events for the RFET while Pedro Munoz remained president.

"We never had any problem with playing in Madrid," Nadal said in Hamburg. "But we can't accept that the president told us 100 times that we were going to decide, and then we don't decide."

The ball moves slower at low altitude which would generally suit clay-court specialists Spain.

"What you can't do now is keep saying we're the clear favourites. We're not playing against number 50 and 60 in the world. Roddick is at number six and Blake is at eight and they have the world's number one doubles team.

"It's going to be a very difficult tie and we'll do everything we can to win but it's going to be very open and anything can happen."

Spain play the United States from Sept. 19-21. They have only beaten the U.S. three times in eight Davis Cup meetings, with all three coming at home, on clay and at low altitude.