Shanghai, 8 October 2011 | AP

Top-ranked tennis star Novak Djokovic has pulled out of the Shanghai Masters because of a back muscle injury, further delaying his return to competition.

Djokovic says he has not yet recovered from the injury he sustained in winning the US Open and which he aggravated while playing for Serbia in the Davis Cup semi-final against Argentina.

“I did another check this week and the results are not good. I need to continue my rehabilitation,” Djokovic said.

The tournament in Shanghai starts on Monday.

Djokovic has already pulled out of the China Open in Beijing.