Anthony Joshua and Wladamir Klitschko take part in a press conference for their Super Heavyweight title fight at Sky Sports Studios. Getty

A heavyweight championship fight in London means an early start in the United States for viewers. In front of a sell-out crowd of 90,000 Saturday at Wembley Stadium, Anthony Joshua faces Wladimir Klitschko for Joshua’s IBF World Championship and the vacant IBO and WBA World Championships live on Showtime Sports at 4:15 p.m. ET.

HBO will air a same-day replay at 10:45 p.m. in all U.S. time zones. The fight is expected to begin at 5 p.m. ET.

The joint partnership between HBO and Showtime is rare but it's not the first time that such a deal has taken place.

“I'm extremely happy and thankful that our respective U.S. TV partners Showtime and HBO reached an agreement,” said Bernd Boente, Managing Director of Klitschko Management Group. “This happened before in the Mayweather vs. Pacquiao and the Lewis vs. Tyson fights and shows you the magnitude of our event at Wembley Stadium on April 29.”

A live stream of the fight can be seen on the Showtime App. In the U.K., the fight can be seen on live on Sky Sports Box Office.

Joshua, who is making his third title defense, is the favorite after winning his first 18 fights. The 27-year-old is expected to have the home crowd on his side.

Klitschko, 41, is coming off an uninspired performance against Tyson Fury in November 2015. The Ukrainian had previously been undefeated for 11 years.

The winner of the fight could be in line to fight undefeated American Deontay Wilder.