World number eight Milos Raonic has withdrawn from the World Tour Finals at London's O2 Arena with a quad injury.
Spain's David Ferrer, who is the first alternate, will step in to play Japan's Kei Nishikori on Thursday afternoon.
The change means Britain's Andy Murray, who is also in Group B, will be eliminated if he loses to Roger Federer this evening.
"You, the fans deserve a great show and I wouldn't step out if I couldn't," tweeted Canadian Raonic.
"I am sorry for the fans. I will do everything to be here next year."
Raonic confirmed in a press conference that he had suffered the injury in the first set of his defeat to Murray on Wednesday evening.
He added: "If I was to step out on court, I'd be walking a tightrope where I can play obviously not at 100%, but with a significant risk of losing up to six to eight weeks if things go badly."
Raonic also lost his opening match in Group B against Roger Federer.
World number 10 Ferrer has appeared in the last four Tour Finals and only narrowly failed to qualify for this year's edition, suffering a decisive defeat to Nishikori in the quarter-finals of the Paris Masters.