By Julien Pretot

PARIS (Reuters) - Novak Djokovic pulled out of the Paris Masters on Friday because of a sore shoulder and apologized to fans who had bought tickets for his quarter-final showdown with French favorite Jo-Wilfried Tsonga.

"Sadly I have to inform you that I have withdrawn from the tournament. I have pushed myself to the limit by playing, and after the match yesterday my shoulder got worse," the Serbian world number one said on his official website (www.novakdjokovic.rs).

"I am very sorry for all of you who bought tickets and wanted to come and watch me play.

The 24-year-old, who pocketed a $1.6 million bonus just for turning up in Paris by virtue of an ATP rule designed to entice top players to take part in the main events, now faces a race against time to get fit for the season-ending ATP World Tour Finals in London from November 20-27.

Having skipped the Shanghai Masters through injury, he suffered a recurrence of a shoulder problem in his semi-final defeat in Basel by Japan's Kei Nishikori last week.

Djokovic's withdrawal means the sixth-seeded Tsonga will take on either American John Isner, or Spain's David Ferrer, the fourth seed, in the Paris semi-finals.

Briton Andy Murray will open proceedings at Bercy on Friday against Czech Tomas Berdych, followed by Roger Federer against Argentine Juan Monaco.

(Reporting by Julien Pretot; Editing by Alastair Himmer and Pritha Sarkar)