Former undisputed heavyweight king Wladimir Klitschko is very familiar with both Deontay Wilder (40-0, 39 KOs) and Anthony Joshua (21-0, 20 KOs).

During the core run of his career, Klitschko had used both boxers as his sparring partners.

Wilder holds the WBC heavyweight crown, while Joshua has the IBF, WBA, WBO, IBO world titles.

Klitschko retired from the sport last year, a few months after suffering an eleventh round TKO loss to Joshua before a record crowd of 90,000 fans at Wembley in London. Many view it was the best performance of Klitschko's career.

Klitschko had Joshua down and hurt in the sixth round, but Joshua eventually rallied in the later rounds to stop the Ukrainian boxer in the eleventh.

A rematch was scheduled for last fall, but Klitschko decided to back away from a second fight and retired instead.

At the moment there are ongoing negotiations to stage a showdown between Wilder and Joshua in the coming months.

The two sides are currently at odds on the money and some of the other details, like the fight location. Both teams are going to continue their talks and hope to reach a deal in the near future.

Klitschko was asked for his opinion on a potential fight between the two undefeated world champions.

"They are completely different fighters. I know both of them. They were both at my training camp. We spent a lot of time with each other. They are completely different fighters," Klitschko told BBC.

"It's very difficult to calculate where Deontay Wilder's punches are coming from. But the strength, the core, the foundation that Anthony Joshua has, in addition to being an Olympic champion, and he has the great amateur experience that Wilder does not have. I'll be happy, super happy if I'm able to see him (Anthony Joshua) succeed. I told him 'the more you shine, the more I shine."