Canada's Milos Raonic withdrew from the ATP Tour Finals on Thursday due to a thigh injury and was replaced by Spain's David Ferrer.
Ferrer, who had been practising at London's O2 Arena as the first alternate, will take Raonic's place against Japan's Kei Nishikori in the final round of Group B matches on Thursday.
The Spaniard only missed out on a place in the season-ending tournament at the last minute after Raonic defeated Roger Federer on his way to the final at the Paris Masters.
However, Raonic lost both of his matches on his debut at Tour Finals and revealed he suffered the injury in Tuesday's defeat to Andy Murray
"I have a slight tear in my quad and a large area of swelling," he said.
"It would be unfair to fans if I was to go out and play a mediocre match. If I was to step out on court I could lose up to six to eight weeks. That was a significant factor in my decision."
Raonic's decision potentially has a significant impact on the qualification hopes of both Nishikori and Murray, who meets Federer in Thursday's later match.
There had been a scenario whereby Murray could lose to Federer and still make the semi-finals if Nishikori had also lost to Raonic.
But, instead of a three-way tie being decided by sets or games percentage, as would have been the case, a player playing fewer matches is automatically eliminated from the race leaving the remaining two players to be split by head-to-head.
Because Murray lost his opening match to Nishikori, he must now beat Federer to stand a chance of going through.
Should Nishikori lose, any win for Murray would be enough, but victory for the Japanese player would mean the Scot must win in straight sets and hope his games percentage is better than either Nishikori or Federer.
Source: AFP