LONDON (Reuters) - Canada's Milos Raonic has withdrawn from the ATP Tour Finals in London owing to injury and been replaced by Spain's David Ferrer.

The 23-year-old lost his first two round-robin matches at the season-ending event to Roger Federer and Andy Murray.

"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. It was a hard decision," Raonic said on Twitter.

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

Raonic, the first Canadian singles player to qualify for the showpiece event, has been replaced by Ferrer and the Spaniard will play Japan's Kei Nishikori later on Thursday.