Juan Martin del Potro of Argentina greets the audience after winning the men's semifinal against Novak Djokovic of Serbia at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Juan Martin del Potro won 2-1 to enter the final. (Xinhua/Yang Lei)

Novak Djokovic of Serbia returns the ball during the men's semifinal against Juan Martin del Potro of Argentina at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Novak Djokovic lost 1-2. (Xinhua/Yang Lei)

Juan Martin del Potro of Argentina celebrates for score during the men's semifinal against Novak Djokovic of Serbia at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Juan Martin del Potro won 2-1 to enter the final. (Xinhua/Yang Lei)

Novak Djokovic of Serbia reacts during the men's semifinal against Juan Martin del Potro of Argentina at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Novak Djokovic lost 1-2. (Xinhua/Yang Lei)

Novak Djokovic of Serbia reacts during the men's semifinal against Juan Martin del Potro of Argentina at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Novak Djokovic lost 1-2. (Xinhua/Yang Lei)

Novak Djokovic of Serbia reacts during the men's semifinal against Juan Martin del Potro of Argentina at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Novak Djokovic lost 1-2. (Xinhua/Yang Lei)

Juan Martin del Potro of Argentina returns the ball during the men's semifinal against Novak Djokovic of Serbia at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Juan Martin del Potro won 2-1 to enter the final. (Xinhua/Yang Lei)

Juan Martin del Potro of Argentina returns a hit during the men's semifinal against Novak Djokovic of Serbia at the BNP Paribas Open on March 16, 2013 in Indian Wells, California, the United States. Juan Martin del Potro won 2-1 to enter the final. (Xinhua/Yang Lei)