MADRID - THE United States face a formidable challenge this weekend when they take on a strong Spanish team, led by world number one Rafael Nadal, in a Madrid bullring for a place in the Davis Cup final against either Argentina or Russia.

It was Nadal, then just 18 and relatively unknown, who sensationally downed Andy Roddick, then ranked number two in the world, to help Spain defeat the US and win only its second Davis Cup trophy in Seville in 2004.

Four years on, he is having the best season of his career, having won Wimbledon, the French Open and an Olympic gold medal and dethroned Roger Federer from the top spot in men's tennis.

He will be helped in the singles by world number five David Ferrer, while 15th-ranked Fernando Verdasco and Feliciano Lopez, ranked 37, will play the doubles against Mike Bryan and Mardy Fish.

Fish is a replacement for Bryan's usual partner, his twin brother Bob, who pulled out with a shoulder injury.

The Bryan twins have played the last 14 Davis Cup doubles rubbers for USA since making their debut in September 2003, and would have been a good bet for a point.

Last week, ninth-ranked James Blake withdrew from the US squad, citing exhaustion, and was replaced by the 20-year-old Querrey, ranked 39.

Sanchez Vicario and his US counterpart Patrick McEnroe agree that Madrid's high altitude, which makes the ball fly faster, could help the Americans.

'The tie will be equal' due to the high altitude, said Sanchez Vicario.

And Nadal is also showing signs of fatigue with his US loss to Andy Murray in the US Open semi-final.

'It's true that I've had three or four days to rest at home ... but I'm not 100 per cent recovered,' he told the Spanish sports daily AS.

'This has been a non-stop year, it's normal that at some moment you deflate.' He predicted 'an open semi-final', which would be his 'the last big effort'.

'After that, I hope to take a real break in order to handle the last section of the season'.

In the other semi-final, Argentina and Russia clash in Buenos Aires, where the Argentines will hope to repeat a 5-0 win they achieved in the 2003 quarter-final.

However, it was the Russians who had the edge the last time the two countries met - in the trophy match 2006, when they edged through 3-2 on carpet in Moscow.

Argentina's world number seven David Nalbandian is confident.

'We are looking good, we have been training hard and are in good form. But it will be a tough encounter and we have to be 100 per cent focused. We must use home advantage,' said Nalbandian.

Skipper Alberto Mancini will also hope for great things from up and coming star Juan Martin del Potro, while Agustin Calleri and Guillermo Canas complete the group.

'Much will depend on the opening day, where we must ensure we get off to a flier,' Mancini said.

Nalbandian insists he is over a rib injury which forced him to lay low for a month earlier in the season.

'I've had a complicated year but now I'm better and finding my rhythm again,' he insisted.

Mancini picked out Igor Andreev, ranked 19, as 'a player who can either come out firing on a very high level or else be very erratic.'

But there is also the threat of sixth-ranked Nikolay Davydenko, who, as Mancini pointed out, 'has already played two finals and hence has a lot of experience in his locker.'

For Del Potro, who will turn 20 next week, this will be his first home Davis Cup action - and he can hardly wait.

'I mustn't let the pressure get to me but rather enjoy it. I hope I can fulfil people's expectations', said Del Potro.

But the Russians are also confident, insists skipper Shamil Tarpischev.

'It's the third time we will have played in Argentina and so we know how things will be'.

Despite the absence of Marat Safin, a title-winner in Paris in 2002 but who has elected to play in the Thailand Open next week instead of for his country, Tarpischev says his squad is very competitive.

'Come Friday night it could be 2-0 to us - or to Argentina. It's finely-balanced.' -- AFP