Not long after suffering a massive upset knockout loss to Andy Ruiz Jr. on June 1, Anthony Joshua got a phone call from a man to whom he will always be linked, and who knew just how he was feeling.

On the other end of the line was Wladimir Klitschko, a former opponent and one of the great heavyweight champions in boxing history.

Klitschko had called to offer a bit of counsel and support. There was a time when Klitschko, like Joshua, had doubters and even, at times, doubted himself, and could have benefited from a similar call.

"I really want him to shine, and his upset against Ruiz reminded me of my loss against Corrie Sanders," Klitschko told ESPN in reference to the late South African southpaw who ended Klitschko's first title reign by shocking second-round knockout in 2003. "We both totally underestimated quality fighters."

Joshua was all ears. He had just begun preparations for a rematch with Ruiz, and at that moment the loss was still very fresh. After losing his three belts, he was being publicly targeted with criticism and faced a nonstop chorus of boxing pundits asking what had gone wrong. Joshua needed a reassuring voice to put things in perspective before the sequel, which will take place on Saturday (DAZN, noon ET with main event beginning at approximately 3:45 p.m. ET) at Diriyah Arena in Diriyah, Saudi Arabia.

"I respect him so much. He has character," Joshua told ESPN. "I respect everyone in boxing, but Wladimir is way up there for me. I take what he says to heart, man. I listen to what he says. I don't just hear it in one ear and out the other."

Klitschko had lived what Joshua was going through. Following two knockout losses in the span of four fights in 2003 and 2004, Klitschko had been left for dead by most fans and media. First it was Sanders, a big puncher with fast hands, who got him. Then, after dominating Lamon Brewster for four rounds, an exhausted Klitschko succumbed in the fifth with a vacant world title on the line.

Klitschko ignored calls from some to retire -- calls that came even from his own brother, fellow former heavyweight champion Vitali Klitschko -- and from there embarked on a historic run in which he won 22 fights in a row, unified three major world titles, dominated the division from 2006 to 2015 and made 18 consecutive defenses, the third-longest streak in the history of the division. He eventually lost those belts by upset decision to Tyson Fury in 2015. Klitschko returned in April 2017 to challenge Joshua, who had won a title after Fury's personal problems forced him to relinquish all of the belts.

Joshua famously stopped Klitschko in the 11th round of their epic fight of the year in front of 90,000 at Wembley Stadium in London, sending Klitschko into retirement.

Former heavyweight world titleholder Anthony Joshua, left, and former heavyweight champion Wladimir Klitschko became friends even before they faced each other in 2017. AP Photo/Richard Drew

Klitschko and Joshua were friends before the fight, and despite an intense battle in which both men were knocked down over the course of 11 rounds, they have remained so since. It's a bit of a different dynamic than other friendships forged in boxing, like the bond between the late Arturo Gatti and Micky Ward. Gatti and Ward didn't know each other until they fought, and they became closer and closer during their famed trilogy, to the point where they were the best of friends by the time it was over; Ward even trained Gatti for his final fight.

Klitschko and Joshua's friendship dates back to 2014. When Klitschko was preparing for a title defense against Kubrat Pulev, he invited then-prospect Joshua to join his training camp in the mountains of Austria as one of his sparring partners.

Klitschko and Joshua sparred about 15 rounds during that training camp.

"While watching AJ winning his Olympic gold medal I called [late Hall of Fame trainer] Emanuel [Steward] and asked if he saw the new bulked-up [Olympic] heavyweight champion," Klitschko said. "I was following AJ since the finals of the Games, and he somehow reminded me of myself in a certain way.

Klitschko was extremely complimentary of the rising boxing star's efforts at the time.

Anthony Joshua, right, defeated Wladimir Klitschko by 11th-round TKO at London's Wembley Stadium in 2017. Dan Mullan/Getty Images

"If he is going to keep doing what he is doing, he will be champion for sure," Klitschko told ESPN during that training camp. "I think he has amazing potential. He needs to learn, but that will come. I think he has the size and the weight. He's built like a modern professional warrior and athlete. He has good technique and good amateur background, which is important."

"He is an Olympic champion, but he doesn't carry himself as Olympic champion like, 'I'm so cool.' He is very humble. He learns. He asks a lot of questions,and he is very open-minded and wants to learn. I enjoyed having him in camp."

After the camp was over, Joshua said he stayed in touch with Klitschko and trainer Johnathon Banks, who became Klitschko's head cornerman following Steward's death in 2012.

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"We've always been in touch, so Wladimir has always had my number and he just reached out to call me," Joshua said of the call he received in August. "I texted him one Saturday morning, and he gave me some of his own tips and point of view.

"When I spoke to the champ, Wladimir Klitschko, he says, 'Anthony, why you focusing on so much running and treadmills and rowing machines? You need to be boxing fit; you need to focus on your boxing.'"

Joshua (22-1, 21 KOs), of England, said the words from Klitschko were not particularly dramatic but had enough impact to make him rethink elements of his preparation. Joshua said he and Klitschko have talked several times since his stunning loss in his American debut at Madison Square Garden to late replacement Ruiz (33-1, 22 KOs), 30, of Imperial, California.

Andy Ruiz Jr. dropped Anthony Joshua four times back in June at Madison Square Garden in New York, to win three heavyweight world titles. Nick Potts/Getty Images

The 30-year-old Joshua said he appreciated that Klitschko offered his advice privately, in contrast to former heavyweight champion and British countryman Lennox Lewis, who questioned Joshua's preparation for the June fight via social media.

Joshua said Lewis' comments upset him. He called Lewis a "clown," and it has led to bad feelings between the two. He said he would have appreciated Lewis calling him to voice his opinion rather than doing so publicly.

"This is why I love Wladimir, because he calls me," Joshua said. "He doesn't go on the internet and say, 'Joshua needs to do this, or Joshua is wrong with this.' He just calls me and says, 'Champ, look, this is what you need to do.' And he does it in a way of respect. Wladimir has been in the ring [with me], so he'll call me.

Throughout his training for the rematch with Ruiz, Joshua said he's been in touch with Klitschko.

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"He tells me where I've gone wrong, but he doesn't do it all over the internet," Joshua said. "He respects my team and he respects me as a person, and that's why he reached out and called me."

Klitschko said he will be rooting hard for Joshua but has no idea what will happen.

"No one knows, and even AJ doesn't really know himself yet -- unless his gut feeling tells him, 'This is me, and I will make it.' I had a similar set of circumstances after the [first] Lamon Brewster fight," Klitschko said. "I was written off by everyone, but my gut told me, 'This is not the end. I will be a record-breaking champion, just wait and watch, guys.'"

Joshua, who arrived in Saudi Arabia a week ago, is aiming to replicate Klitschko's revival and said he spoke most recently to him about two weeks ago. He said he received more simple, but useful, advice as the fight drew near.

"He said, 'Stay off your phone, no social media, focus on yourself,'" Joshua said. "I needed to do something for him -- I was going to send him some gloves [to auction off] for his charity -- and he said, 'Just wait until after the fight. Right now just focus on your fight, stay off your phone and don't listen to any outside stuff.'"

Other than a few phone interviews with American reporters, Joshua said he is once again taking Klitschko's advice. Now it's on Joshua to follow Klitschko's path as well.