Tyson Fury and Oleksandr Usyk have signed contracts for an undisputed heavyweight championship fight in Saudi Arabia, multiple sources told ESPN on Friday.

The fight could take place on Dec. 23 or sometime in January, sources said. First, Fury must handle business when he meets former UFC champion Francis Ngannou on Oct. 28 in Saudi Arabia on ESPN+ pay-per-view.

Fury-Usyk will crown boxing’s first undisputed heavyweight champion in the four-belt era and is a rare meeting between heavyweights recognized as two of the best pound-for-pound fighters in the world.

Fury and Usyk were in talks to fight in April at London’s Wembley Stadium before negotiations collapsed at the 11th hour.