Danish WBC super-middleweight champion Mikkel Kessler has pulled out of the Super Six tournament because of an eye injury.
Wednesday's announcement means next month's bout between Kessler and American Allan Green in Denmark has been called off.
Kessler (43-2) beat Britain's Carl Froch in April to win the WBC title.
Kessler said: "I have a few problems when I go down the stairs and when I close one eye, I don't see double. It means that I had to give up my big dream, but I am not throwing the towel in the ring."
Promoter Kalle Sauerland said it was "not bad news but it is a bombshell."
Christian Meyer, the managing director of promoters Team Sauerland, said a medical check on Kessler in Germany on Tuesday "confirmed what was recorded by other specialists."
Meyer added: "The doctor confirmed that only a proper rest - so not much of a treatment - simply rest from boxing activity would lead to a full recovery."
Verbal agreement
Sauerland said he had "a verbal agreement" for a fight next year between the 31-year-old Kessler and the winner of the Super Six.
The Super Six tournament gives six of the best 168-pound boxers in the world three fights each, with points awarded. The four fighters with the most points advance to semi-finals, with a championship bout next year.
American Andre Ward is the only fighter to win his first two bouts - including one against Kessler - and leads the tournament with four points.