* Ferrer battles back from a set down
* Czech Republic await in November final (Adds result of second semi-final, quotes)
GIJON, Spain, Sept 16 (Reuters) - Holders Spain marched on towards a fourth Davis Cup triumph in five years when David Ferrer overcame the big-serving John Isner to help beat the United States 3-1 and set up an away clash with the Czech Republic.
World number five Ferrer had to fight from a set down on the clay in the northern Spanish coastal city of Gijon, coming through 6-7 6-3 6-4 6-2 against the 10th-ranked Isner in exactly three hours for his 16th victory in 16 singles rubbers on clay.
"It was a very tough, very difficult tie but we are really happy as we are in another Davis Cup final, something that is extremely hard to achieve," Ferrer said in an interview with Spanish television.
"Now we just need to enjoy it and rest and prepare for the final," added the 30-year-old, who has won five titles this year and reached the U.S. Open semi-finals this month.
Favourites Spain will seek a sixth Davis Cup triumph since their first in 2000 and will hope for a repeat of their last meeting in the final against the Czechs when the pair meet in November.
Spain thrashed the Czechs 5-0 in the 2009 title match, their most recent meeting. Competition rules dictate that because it was in Spain the next must be in Czech Republic.
World number six Tomas Berdych gave the Czechs an unassailable 3-1 lead in Buenos Aires on Sunday when he beat Argentine replacement Carlos Berlocq in the first reverse singles following the withdrawal through injury of eighth-ranked Juan Martin del Potro.
Berdych and his team mates will likely choose hard courts for the Nov. 16-18 final as Spain are extremely tough to beat on their favoured clay and Spanish captain Alex Corretja said his players would just have to adapt.
"Here you play in front of your home crowd and on your best surface and you pick everything," he said on the Davis Cup website (www.daviscup.com).
"We'll wait and see what the opponents want and be ready for that and try to do our best," he added.
Ferrer said the Spanish were hoping talisman Rafa Nadal, who has not played since Wimbledon, would have shrugged off a knee injury in time to feature.
"Spanish tennis is blessed with very good players, led by Rafa Nadal, and we hope he recovers soon because he is fundamental for us."
Isner told Spanish TV before Sunday's match Ferrer was probably his favourite player on the tour for his work ethic and fighting spirit but it was the American who battled back from a break down to take the opening set 7-3 in the tiebreak.
One U.S. fan held up a card counting each of Isner's booming aces - he powered down 16 in the match to Ferrer's five - but the American also made 70 unforced errors to Ferrer's 32 and he was visibly tiring as his challenge faded in the fourth set.
He saved a match point on his serve to stay in the match but it was merely delaying the inevitable as Ferrer sealed victory the next game when Isner went wide with a limp backhand.
Ferrer flung his racket into the crowd before he, Isner and Corretja and U.S. captain Jim Courier shook hands at the net. Shortly after it was announced that Nicolas Almagro and Sam Querrey would not contest the dead final rubber.
"I think it was a great battle between two of the best teams in the world," Courier said.
"Spain is the high watermark in Davis Cup in the last 15 years and we knew it was going to be a very tricky tie for us and we were close," added the former world number one.
Top-ranked U.S. pair Bob and Mike Bryan had kept their team's hopes of upsetting the Spanish alive when they fought off a brave challenge from Marcel Granollers and Marc Lopez to win their doubles on Saturday.
Ferrer and Almagro beat Querrey and Isner respectively in Friday's singles.
Spain's latest victory extended their unbeaten run at home to 24 ties dating back to 1999 when they lost 3-2 to Brazil in Lerida.
The U.S. are the most successful Davis Cup nation with 32 titles, while Spain have dominated the competition over the past decade, with four titles since their first one in 2000. (Writing by Iain Rogers in Madrid, editing by Tom Pilcher and Pritha Sarkar)