6:05 PM ET

Anthony Joshua and Oleksandr Usyk will run it this summer.

Usyk was given permission to leave Ukraine and begin training for the title match after crossing the border to Poland, according to sources.

Russia invaded Ukraine and the unified champion joined a territorial defense battalion. During the war, martial law was instituted and men who were fit to serve were banned from leaving the country. The opportunity to fight George Kambosos for the undisputed lightweight championship on June 5 in Australia was passed up by Usyk's friend, Vasiliy Lomachenko.

Usyk will head into a second fight with Joshua after he soundly defeated the Englishman in September. Joshua exercised his contractual right to a second fight with financial terms already agreed upon.

Usyk told CNN earlier this month that he didn't know when he would return to the ring.

Usyk said that his country and his honor were more important than a belt.

Eddie said he would wait as long as Usyk needed. Plans are underway to get a date and location for the second fight between Joshua and his opponent.

Usyk is a boxer. The Olympic gold medalist won three belts in his third fight in the division after a dominant run at cruiserweight.

Usyk will be installed as a heavy favorite to retain his belts and possibly meet the winner of the April 23 fight between Tyson Fury and Dillian Whyte later this year.

Joshua is an Olympic gold medal winner. He won his titles back with a decision victory in the second fight after being knocked down in the seventh round.