Joshua defeated the Ukrainian at Wembley in 2017 in what proved to be the final fight of Klitschko’s 21-year career.
For a then-undefeated Joshua, it represented a 19th consecutive win to start his career, adding the WBA and IBO heavyweight titles to the IBF belt he already held.
The British boxer has since been beaten by both Andy Ruiz Jr and, most recently, Klitschko’s compatriot Oleksandr Usyk, but the 32-year-old still ranks the younger Klitschko brother as his toughest in-ring foe.
“The hardest opponent I have come up against would be Wladimir Klitschko – definitely,” Joshua said at an event at the Oxford Union.
“[It was] the passing of the guard. The young lion versus the old lion. At the time I fought him, I thought it was definitely too early but it was his last fight.
“So if I didn’t fight him then, it would have been too late. It was risk versus reward and I thought boxing needed it.”
The encounter set a new post-war record with 90,000 spectators inside Wembley Stadium to watch Joshua win with an eleventh round TKO.
Joshua suggested afterwards that he would accept a rematch, but Klitschko confirmed he would be bowing out of boxing a few months later.
The Ukrainian had managed to knock down the younger fighter in the sixth round, but Joshua managed to endure and battle back to secure victory.
“Sometimes, due to a lack of experience, we make it harder than it sometimes needs to be,” Joshua recalled from his bout with Klitschko.
“He had definitely more knockouts on his record than I have fights and knockouts combined. He is experienced and very strong and it was a tough fight.
“Before that stage, I was knocking guys out within six rounds and seven rounds.
“Wladimir took me 11 rounds – somewhere I had never been before. He was my toughest, for sure.”
Joshua is expected to attempt to win back the unified heavyweight titles from Usyk this summer, with a likely fight date in late July.
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