The Barcelona playmaker warned against any assumptions of an easy encounter for the European champions, saying he felt there was "perhaps more pressure on Spain than the U.S."
"I've seen the strong team the Americans have been evolving ... and when we beat them 1-0 in a friendly last year they were a difficult opponent," Xavi told a news conference on Tuesday.
"They will be a very competitive rival. We may be the favourites but they will not be an easy opponent to beat. If Spain are favoured then the United States really have nothing to lose."
Spain, who set a world record of 15 successive international wins when they beat hosts South Africa in their last group game on Saturday, have sauntered through to the last four after also beating Iraq and New Zealand in their section.
"We don't think we are that much superior to the United States," said coach Vicente del Bosque.
"It took a lot of hard work to get through a supposedly weak group."
Wednesday's semi-final at the Free State Stadium in Bloemfontein is followed on Thursday in Johannesburg by a match between Brazil and South Africa.
The final is on Sunday.
