(Reuters) - With Andy Roddick retired, Mardy Fish unavailable, and Spain apparently unbeatable at home on clay - the United States look distant second favourites when the two nations clash in the Davis Cup semi-final in Gijon starting on Friday.

John Isner of the U.S. hits a return to Philipp Kohlschreiber of Germany during their match at the US Open men's singles tennis tournament in New York, September 2, 2012. REUTERS/Bill Kostroun

Then again, few fancied their chances of beating a Switzerland team that included Roger Federer in round one and France in the quarter-finals led by the big-serving John Isner.

Argentina will also start as favourites in their semi-final against the Czech Republic in Buenos Aires with Juan Martin del Potro and Juan Monaco shouldering their nation's hopes of winning the title for the first time.

The Czechs, however, boast in-form Tomas Berdych in their ranks and will be looking to repeat their victory over Argentina in Buenos Aires during the 1980 campaign which culminated in them claiming the huge trophy for the only time.

U.S. captain Jim Courier's tenacity seems to have rubbed off on his side this year and while Spain are overwhelming favourites as they chase a fourth title in five years, there are still a few reasons for optimism for the Americans.

When nations lose their top players captain's hope that one of the underlings steps up to the plate and in the case of Spain and the United States, their wishes have been granted.

Spain have had to make do without the injured Rafa Nadal this season but world number five David Ferrer and Nicolas Almagro underlined the Iberian nation's depth as they cruised past Kazakhstan and Austria to reach the semi-finals.

Likewise, Isner has assumed the leaders' role for Courier's side in the absence of former stalwart Roddick.

World number 10 Isner has a perfect 4-0 record in Davis Cup singles this season, taking the scalps of Federer on clay in Fribourg and Jo-Wilfried Tsonga, also on the dirt, to close out the quarter-final in Monte Carlo.

"John's fresh and focused. He's beaten some substantial players this year," Courier who played 14 ties for the U.S. and sealed the 1992 title against Switzerland, said on the United States Tennis Association website.

"He's going to have to be fresh and focused to stand a chance against Ferrer and Almagro. Those guys, you saw what I saw at the French Open. Those guys were playing some serious clay-court tennis.

"John's been incredible in Davis Cup this year. He's incredibly excited about Gijon."

WARY SPAIN

Ferrer will open the tie against Sam Querrey on Friday defending a 14-0 singles record in Davis Cup on claycourts so Isner will almost certainly be playing catch-up by the time he takes to the Parque Hermanos Castro court against Almagro.

Spain captain Alex Corretja said any team with Courier in charge would be dangerous.

"It's not going to be easy by any means. They have a captain like Courier with a great deal of experience," he said.

"They have the best doubles team (Mike and Bob Bryan) in the world and even though they are our opponents you have to admit they are great to watch."

For the Americans to pull off a shock and end Spain's run of 23 consecutive home victories in Davis Cup the world number one Bryans will have to earn their point when they take on Marcel Granollers and Marc Lopez on Saturday.

Argentina, the only one of the semi-finalists without a title will believe that this could finally be their year, having lost all four finals they have contested including last year's title clash against Spain.

However there is a question mark about the fitness of eighth-ranked Del Potro, who has a left wrist problem.

"What I need (to do) to recover is rest as my doctor has already told me but I'll try to leave that for later on," Del Potro said after practising with the team at Parque Roca, the venue for the tie.

"It's something beautiful to be able to play here in Argentina and I'm going to make the effort for the crowd and try to give my best," added the 2009 U.S. Open champion, who had surgery on his right wrist in 2010.

(Editing by Tom Pilcher)

• Please wait.

• Rachel owes her rich vein of form to change in technique
• Murray out to crash Big Three's Wimbledon party
• Tales up to French starting berth for final NZ test
• Williams has mental edge over rest at Wimbledon
• Heathcote back at flyhalf for Scotland
• Brotherly love leads Leiua to Samoa
• Light at the end of the tunnel for test Lion Corbisiero
• FIA accuses Mercedes of gaining from illegal test
• Australia's Folau in uncapped trio to face Lions
• Hurdler Pearson to return from injury next week