Del Potro, who reached a career-high ranking of number three earlier this season, injured his knee while competing at the Shanghai Masters last month.
"It's frustrating for me not to be able to compete in London," said Del Potro.
"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."
Nishikori, 28, will be making his fourth appearance at the season finale, having reached the semi-finals in 2014 and 2016.
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)