Argentina's Juan Martin del Potro has withdrawn from the season-ending ATP Finals in London and will be replaced by Japan's Kei Nishikori.
World number four Del Potro is still sidelined with the right knee injury he 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 event.
"It's frustrating for me not to be able to compete in London," said Del Potro.
The Argentine, 30, last played in the tournament in 2013 but has been plagued by injuries since.
"It's a very special tournament and I've tried everything possible to get my knee better," he added. "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."
The ATP Finals run at the O2 Arena from 11-18 November.