Djokovic pulls out of Davis Cup

U.S. Open champion Novak Djokovic pulled out of Serbia's opening singles match against David Nalbandian of Argentina on Friday in the Davis Cup semifinals because of back pain.

Viktor Troicki will replace Djokovic, who returned to Serbia on Thursday after beating Rafael Nadal on Monday in the U.S. Open final.

The top-ranked Djokovic also pulled out of Serbia's opening match of the semifinals last year against the Czech Republic because he needed rest after the U.S. Open. He later led Serbia to its first Davis Cup title.

In Friday's other singles match, Janko Tipsarevic will face Juan Martin del Potro.