A back injury has forced world number one Novak Djokovic to pull out of the China Open and it could see him skip the Shanghai Masters that begins in two weeks. The injury also caused him to quick during a Davis Cup semifinal match against Argentina's Juan Martin del Potro, but Djokovic, who won his third Grand Slam at the US Open earlier this month, says the injury is not serious and requires only rest. Tournament organizers said the defending champion withdrew because the latest medical checks show the injury he sustained while winning the US Open has not yet healed. The tournament in Beijing starts on Monday. Djokovic has lost only one completed match this season and has won three Grand Slams. BM