Milos Raonic
Raonic has been replaced by David Ferrer for the final round robin match against Kei NishikoriGetty Images

Milos Raonic has pulled out of the ATP World Tour Finals with injury to leave Andy Murray's hopes of reaching the semi-finals hanging by a thread in London.

The Canadian has withdrawn with a "quad tear" and will be replaced by David Ferrer for the final round robin match against Kei Nishikori.

Ferrer cannot qualify even if he beats Nishikori but it means that Murray must overcome Roger Federer in the evening session to stand any chance of reaching the last four.

The British No.1 could previously have lost to Federer in three sets and still qualified for the knock-out phase but must now beat the six-time champion to keep his hopes alive in the English capital.

"I am sad to announce that I have to withdraw from the rest of this competition, due to a low grade tear in my quad," Raonic wrote on Twitter.

"It was a hard decision. You, the fans deserve a great show & I wouldn't step out if I couldn't. I am sorry for the fans. I will do everything to be here next year."