LONDON (Reuters) - Argentina's Juan Martin del Potro has withdrawn from the season-ending ATP Finals and will be replaced by Japan's Kei Nishikori, organisers of the London event said on Saturday.
World number four Del Potro is still sidelined with a right knee injury sustained in Shanghai last month.
Nishikori will join Novak Djokovic, Rafael Nadal, Roger Federer, Alexander Zverev, Kevin Anderson, Marin Cilic and Dominic Thiem at the elite eight-player event.
"It's frustrating for me not to be able to compete in London," Del Potro, who last played in the event in 2013 since when he has been plagued by injuries, said in an ATP statement.
"It's a very special tournament and I've tried everything possible to get my knee better. The rehabilitation is making good progress, but I need more time.
"Of course, it's disappointing for me right now, but I had a very good season overall."
(Reporting by Martyn Herman, editing by Ed Osmond)