Argentina's players celebrate after beating Serbia in their Davis Cup World Group tennis semi-final match in Belgrade
Argentina's players celebrate after beating Serbia in their Davis Cup World Group tennis semi-final match in Belgrade September 18, 2011. Reuters

Argentina beat holders Serbia 3-2 in their Davis Cup semi-final after world number one Novak Djokovic retired with a rib and lower back injury 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 and then retired, giving Argentina an unassailable 3-1 lead before Janko Tipsarevic beat David Nalbandian in the dead rubber.

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.

I feel very disappointed to end the tie in this way, I tried although I was only 60 percent fit and I went into the match knowing there was a risk of aggravating the injury which I first felt at the U.S. Open, Djokovic told a news conference.

We knew my condition was not good but we believed that even so I would have a better chance against Del Potro than my team mate Viktor Troicki would, at the end of the day it was my decision and it backfired.

I am not saying I would have won if I had been 100 percent fit because Del Potro played at a very high level today and never in my professional career did I struggle with my return of serve as I did today.

The important thing now is to determine the extent of my injury and how long it will take me to recover, I was able to battle through the pain in the U.S. Open final but not today.

Games went with serve in a tense first set with no break points on either side. Del Potro clinched the set after winning three of the last four points in the tie break.

Clearly lacking the spark that took him to three grand slams and seven other tournament wins this season, Djokovic showed signs of nerves after making a flurry of unforced errors in front of a supportive home crowd.

Del Potro finally broke the Serb's serve in the second game of the second set and was in complete control when Djokovic decided he could not carry on.

I feel sorry for Djokovic because he has had an incredible season and I hope he recovers quickly, Del Potro said.

He must have felt a little tired after returning from New York, he made a very good challenge today but I was very confident with my serve and forehand and delighted that we have gone into the final.

It was a great match to play in front of his crowd who were really fantastic, I really enjoyed playing here and we are going to Spain knowing that they are the favourites but hopeful that we can win finally the Davis Cup.

Three-times runners-up Argentina will play Spain in the final after the 2009 champions won both reverse singles on Sunday to complete their 4-1 win over France.