Without Djokovic, Serbia trails Argentina 2-0

BELGRADE, Serbia (AP) -In the absence of the top-ranked Novak Djokovic, David Nalbandian and Juan Martin del Potro won their opening singles Friday to clinch a commanding 2-0 lead for Argentina over defending champion Serbia in the Davis Cup semifinals. Djokovic pulled out of the opening singles against Nalbandian because of back pain, four days after beating Rafael Nadal in the U.S Open final, and was replaced by Troicki who lost 6-4, 4-6, 6-2, 6-3 on Friday. In the other singles match Friday, Del Potro beat Janko Tipsarevic 7-5, 6-3, 6-4. Argentina needs one more point to reach its first final since 2008. The doubles are played Saturday.