Ruiz, who was born in the United States but fights as a Mexican, caused one of boxing's biggest upsets in NY in June when he handed Joshua a first defeat to win the WBA, IBF and WBO crowns in NY.
"We've got to see where he's at because all the pressure is on him".
Because of the late change in opponent in June, both the difference in height between Miller and the shorter Ruiz and the limited time to find adequate sparring partners were cited as excuses for Joshua.
"But I want him to win because I know what he's put into it, how much it means to him and because he's a mate".
And Joshua said fans should expect a third fight between them.
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'I think his last few fights have showed that his chin is a bit suspect, no doubt about that.
He says it was clear Ruiz Jr. was winning the first fight as he had only given one round to Joshua.
"He's been on my mind five weeks prior to June 1 because that's when he came in as my replacement, and he will be on my mind forever", Joshua said. It might be a little harder than the first time. Someone said to me that it would be massive for Matchroom to get hold of the belts again.
Ruiz's performance in NY has made bettors think twice before lumping their cash on the Brit and while Joshua is again the favourite and has reportedly looked more determined than ever in his camp in Sheffield, he has the added pressure of knowing this fight could make or break his career.
"He saw something that he's never seen before". Yet, it would be a long road back to the top to try and become the heavyweight champion again.