| Juan Martin Del Potro secured the crucial point for Argentina when Djokovic retired [GALLO/GETTY]
Argentina took an unassailable 3-1 lead over Davis Cup holders Serbia in their semi-final after world number one Novak Djokovic retired hurt against Juan Martin Del Potro on Sunday.
Del Potro was leading 7-6 3-0 when Djokovic fell to the ground with a scream of pain, appearing to have twisted his ankle as he blazed a forehand wide.
After he was helped to the bench by captain Bogdan Obradovic and his team mates, a tearful Djokovic embraced Del Potro and received a standing ovation from the 18,000 home crowd in the Belgrade Arena.
Three-times runners-up Argentina will play Spain in the final after the 2009 champions also took an insurmountable lead against France on Sunday.
Rafa Nadal proved he had put his exertions at the U.S. Open firmly behind him when he thrashed France's Jo-Wilfried Tsonga 6-0 6-2 6-4.
The burly Tsonga had helped France to victory in Saturday's doubles to keep the best of five tie alive at 2-1 but was powerless against Nadal's relentless claycourt game as the world number two claimed his 17th win in 18 Davis Cup singles matches.
"Simply put, Rafa was just too good this weekend,'' Tsonga said.
"He's the best player ever on clay court, I think, he's practically unbeatable on this surface and today we didn't create the exception"
"He's the best player ever on clay court, I think, he's practically unbeatable on this surface and today we didn't create the exception."
The victory gave Spain, who are bidding for a third title in four years, an unbeatable 3-1 lead ahead of the remaining singles rubber in which Fernando Verdasco faces Richard Gasquet.
Nadal has got his wish after saying he would prefer to play Argentina in the final as it would mean Spain would be at home and able to choose the venue and surface.
He also reiterated his criticism of the Davis Cup format, which he argues puts too much strain on players already committed to a packed ATP Tour calendar.
"It's always nicer to play at home and the court factor is very important," he said of the final in an interview with Spanish television.
"Another reason is that I have more friends in Argentina than in Serbia," the Mallorca native said.