Juan Martin Del Potro of Argentina returns a shot to Novak Djokovic of Serbia during their Davis Cup World Group tennis semi-final match in Belgrade September 18, 2011. Djokovic retired due to injury in the second set against Del Potro. [Photo/Agencies]

BUENOS AIRES - Argentina's Juan Martin Del Potro on Sunday celebrated his country's qualification to the Davis Cup final for the fourth time in history.

"I am happy, though it is a shame to finish this way because it was probable that I could have won. We didn't deserve to win due to this. I am Djokovic's friend and I feel bad because he made this great effort and finish like that. He is a great player," said Del Potro about world number one Novak Djokovic, who retired his game due to injury.

Del Potro was leading by 7-6, 3-0 on Sunday when Djokovic retired the game played in Belgrade Arena, sending Argentina to the Davis Cup final against Spain on December 2 to 4.

The Argentine player told a sports television channel that "we always believed we could win" the series against the defending champion. Argentina has never won the Davis Cup.

Argentina has played three finals before: in 1981 against the United States in Cincinnati (1-3), in 2006 against Russia in Moscow (2-3) and in 2008 against Spain in Mar del Plata (1-3).

Juan Martin Del Potro of Argentina reacts after beating Novak Djokovic of Serbia during their Davis Cup World Group tennis semi-final match in Belgrade Sept 18, 2011. [Photo/Agencies]