Japanese Kei Nishikori has replaced injured Argentine Martin del Potro in the season-ending ATP Finals in London, starting November 11.
Del Petro pulled out of the elite eight-man tournament due to a right knee injury.
The 30-year-old reached a career-high ranking of number three earlier this season but injured his knee while competing at the Shanghai Masters last month.
Novak Djokovic, Rafael Nadal, Roger Federer, Alexander Zverev, Kevin Anderson, Marin Cilic and Dominic Thiem have already qualified for the tournament.
Nishikori, 28, will be making his fourth appearance at the season finale, having reached the semi-finals in 2014 and 2016.
World number two Rafael Nadal's participation is also in doubt. The Spaniard suffered a knee injury at the US Open, forcing him to spend seven weeks on the sidelines.
He attempted to return at the Paris Masters, but minutes before he was due out on the court, pulled out of the competition with a stomach problem.
Defending champion Grigor Dimitrov of Bulgaria failed to qualify, while South African Kevin Anderson will be his debut in the event.