LONDON -- Former three-time heavyweight world titleholder Vitali Klitschko is the mayor of Kiev in his native Ukraine, but always takes a brief break from his busy and intense political life to be at younger brother Wladimir Klitschko's side for the final days before one of his fights.

Vitali Klitschko, one of the leaders for government reform in Ukraine since vacating his world title and retiring in 2012, will also be in his brother's corner, as always, come Saturday (live on Showtime at 4:15 p.m. ET, tape delayed on HBO at 11 p.m. ET/PT).

Wladimir Klitschko (64-4, 54 KOs) is one fight removed from losing his unified titles to England's Tyson Fury 17 months ago and will challenge British star Anthony Joshua (18-0, 18 KOs) for his belt in an attempt to join Vitali as a three-time heavyweight titleholder in a fight that has captivated the British public.

Vitali Klitschko, left, believes his younger brother Wladimir has too much experience for titlist Anthony Joshua and will knock him out as he looks to reclaim a portion of the heavyweight title. John Phillips/UK Press/Getty Images

It is perhaps the biggest fight in British history. Wembley Stadium is sold out to the tune of a British boxing record crowd of 90,000 and Vitali knows it's a tremendously important fight to his brother, whose hopes of carrying on in a meaningful way at age 41 rest largely on regaining the title after Fury ended his 9½-year title reign, 11-year unbeaten streak and stopped him at 18 consecutive defenses.

"This fight is very important for Wladimir personally," Vitali said on Thursday following the final prefight news conference. "Can you imagine for more than 10 years (he was) fighting as world heavyweight champion, beat everyone. And Wladimir I can say is smart. Fast, great technique, great conditioning, great experience. In (the) last fight he just was not focused. It's loss for him, the title."

For this fight, however, Vitali said his brother is as focused and prepared as he has ever seen him and that the focus and overwhelming experience advantage will be too much for the 27-year-old Joshua.

"I never saw my brother so concentrated before this fight. Right now I know Wladimir is ready," he said. "Joshua has great fight record. Every opponent he knocked out. But Joshua never have (an) opponent so high in quality as Wladimir. That's why this will be very interesting fight and not easy for Joshua. And that's why my predictions is Wladimir knocks him out.

"I saw Joshua (work out) yesterday. I can see many small things. I watch him. My personal opinion is he is (too) heavy. Wladimir was never ever in such great shape. Preparation was without injuries. Everything was perfect."

The 45-year-old Vitali Klitschko (45-2, 41 KOs), who was never knocked down in his career and lost only to Chris Byrd because of a torn rotator cuff and on cuts to Lennox Lewis, walked away from boxing after stopping Manuel Charr in the fourth round of a title defense in September 2012. He retired from the ring to focus on public service in Ukraine, where he has fought hard against government corruption. But he always has made time for his brother's fights and said he will relish a victory as much as Wladimir will if he wins.

"If Wladimir wins it's exactly the same celebration as if it was me. We are very close," he said. "And if Wladimir loses, it is disaster for me. If he wins it's a huge celebration." If Wladimir loses, Vitali said he thinks it would be time for his brother to join him in retirement.

"It's the time to think about stopping and not fighting anymore," Vitali said of a loss. "His future depends on this fight. If Joshua loses this fight he is still young, he has time. He can try to become world champion again. My brother does not have a second chance. That is why we expect him to be ready and, of course, have a good result."