The Brit was stunned back in June by the Mexican-American and now faces the daunting prospect of an immediate rematch.
Joshua will know the ramifications if he loses and it will involve a long road back to the top with Deontay Wilder and Tyson Fury set to face off in at least one more fight to start next year – while a third fight between the pair is already pencilled in for the summer.
Ruiz meanwhile could conceivably kick back and wait for the winner in an undisputed fight should he prevail once more.
Here’s everything you need to know about the fight.
When is Joshua vs Ruiz 2?
The rematch will take place on Saturday, December 7.
What time are the ring walks?
The ring walks are set for 8.45pmpm GMT.
How can I watch the big fight?
You can watch the fight on Sky Sports Box Office, with coverage beginning at 4pm GMT. It will cost £24.95.
Is there a live stream?
Yes, you can watch a live stream through the Sky Sports Box Office app and website on mobile devices by ordering here.
The NOW TV Smart Stick or your NOW TV box will also allow fans to order the fight.
While US fans can stream the fight through DAZN on various platforms, including Chromecast, iOS, Android, Playstation 4 and Xbox One. A one month subscription will cost $19.99, while a year is available for $99.99.
How else can I follow the fight?
If you’re unable to get in front of a TV, you can follow the fight with live updates in the Indy Sport live blog.
The Main Event and Undercard
Andy Ruiz vs Anthony Joshua (WBA, IBF, WBO heavyweight world title)
Alexander Povetkin vs Michael Hunter (heavyweight)
Dillian Whyte vs Mariusz Wach (WBC interim heavyweight title)
Filip Hrgovic vs Eric Molina (WBC international heavyweight title)
Mahammadrasul Majidov vs Tom Little
Joshua 2/5 (KO: 4/5, Points: 4/1)
Ruiz 2/1 (KO: 5/2, Points: 14/1)