Ruiz is expected to tip the scales at roughly the same as the first fight at the official weigh-in in Riyadh on Friday, in contrast to Joshua, who could come in up to half a stone lighter.
This will be the second meeting for the two boxers after predicted underdog Andy Ruiz beat fan favorite Anthony Joshua back in June.
The former foes are now friends and Klitschko has urged Joshua to avoid distractions after undertaking a more-boxing focused training camp for Saturday's rematch in Saudi Arabia.
"I am not here to put on a show, I am here to win", he said.
"There's no way that I'm going to let these belts go", he said.
Andy Ruiz Sr promised his son would deliver for a second time.
The Saudis faced intense diplomatic fallout over last year's murder of dissident journalist Jamal Khashoggi in the conservative kingdom's consulate in Istanbul.
Crown Prince Mohammed, who may attend the fight, has masterminded an economic overhaul programme, called Vision 2030, which aims to make Saudi Arabia less reliant on oil. James "Buster" Douglas ballooned in weight after stunning Mike Tyson in Tokyo in 1990 and barely trained for his first defence against Evander Holyfield, rolling to a dismal third-round defeat.
But Joshua knows he was beaten fair and square by Ruiz, who was under-estimated by many boxing pundits because of his slightly chubby appearance.
"I'm going to be the smarter fighter on the night".
Deontay Wilder and Tyson Fury are backing Ruiz Jr to produce another underdog triumph and Dubois admitted he can't pick a victor.
He said: "When I regain those belts".
"I'm thinking like my 16th fight [vs Charles Matin, 2016, for IBF title] - hungry, determined and focussed not eh goal."And Bruce suggest this identification gives the Brit strength.He adds: "This is a big point."When you give something a title, that is powerful stuff". I went to the venue and it is perfectly designed. "It's a time to keep that challenger mindset and move on to the next target". "I will walk in with confidence and I want everyone to feel the confidence that I will".
"I know AJ's going to come with a different game plan, I know he is prepared and motivated", he said. "I'm ready for whatever comes my way". "I will die trying and do anything that is possible to get that victory".