Kei Nishikori will appear at the ATP Finals after Juan Martin del Potro withdrew due to a fracture to the patella of his right knee.
Nishikori's hopes of featuring in London appeared to have gone with defeat to Roger Federer in the Paris Masters quarter-finals on Friday.
However, Del Potro has pulled out of the Finals due to the injury he sustained at the Shanghai Masers in October and the Japanese has been awarded his place.
"It's frustrating for me not to be able to compete in London," said Del Potro, who reached a career-high ranking of number three earlier this season.
"It's a very special tournament and Ive 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."
Nishikori started the year competing in ATP Challenger tournaments after returning from five months off with a wrist injury and expressed his delight at ending his season on a high note.
The 28-year-old said in a tweet: "First of all I want to wish @delpotrojuan a speedy recovery. He had an amazing year.
"Second I am very excited to join the London field. I started my year with challengers and missed the Aussie Open and Indian Wells.
"Very proud of the way the year turned out."