Roger Federer has been forced to withdraw from his final with Novak Djokovic at the ATP World Tour Finals.

The world number two explained to fans that he was not at peak fitness following his intense semi-final victory over Stanislas Wawrinka on Saturday night.

Federer's withdrawal has seen Djokovic clinch his third consecutive year-end title in London.

Following the announcement, the ATP confirmed that Andy Murray would be stepping in to replace Federer for an exhibition against Djokovic.

"I hope you understand I wanted to come out personally and excuse myself for not playing," Federer told those in attendance at the O2 Arena. "I can't compete without a back at this level."

Federer ends the season with five trophies to his name and a winning record of nearly 87%.