Lewis, 55, famously hung up his gloves in 2003 after beating Klitschko, 50.
But the pair made their peace with one another and have remained firm friends to this day, having kept in touch ever since they last shared the ring together.
Much has passed since that memorable night at the Staples Center in Los Angeles all those years ago.
Klitschko is the current Mayor of Kyiv where he has spent the past few months fighting to defend the capital city amid the ongoing Russian military invasion of Ukraine.
And Lewis, in typically classy fashion, posted a touching tribute in a nod to his former opponent.
He wrote: “Normally on June 21, I would do a fight anniversary post of my fight with Vitali Klitschko, but today I simply want to salute him.
“He is in the biggest fight of his life and the sacrifices he has made to serve his fellow Ukrainian citizens is not just commendable, it’s admirable.
“It takes a lot of courage to step into a boxing ring, and as fighters, we know that every fight could be our last, but the courage to stand up for your country and fellow citizens like he has takes a different level of courage and love.
“So on this June 21, I salute you my friend Vitali.
“May God keep you safe and bless you with peace, love and happiness. Bless.”
To which Klitschko simply replied: “Thank you.”
Klitschko won 45 out of his 47 fights overall during his 16-year professional career, including 41 knockouts in total.
He was inducted into the International Boxing Hall of Fame in 2018, with his brother Wladimir Klitschko following suit four years later.
Speaking to Bleacher Report in 2011, he said: “I never met such a strong opponent as Lennox Lewis. I never looked so horrible after a fight like in that fight. And definitely it was one of the hardest fights in my career.
“And I’m very appreciative as Lennox gave me a chance to prove my skills against the strongest boxer all time.
“And I have a lot of experience as amateur, as a professional, and I can definitely say Lennox was (the) hardest fight in my career.”