Against whom did heavyweight boxer Vitali Klitschko say was the hardest fight of his career?

Cornelius Johannes "Corrie" Sanders (7 January 1966 – 23 September 2012) was a South African professional boxer who competed from 1989 to 2008. He won the World Boxing Organisation (WBO) heavyweight title in 2003 after knocking out Wladimir Klitschko in two rounds, which was considered one of the biggest upsets in heavyweight boxing history; "The Ring" magazine named it the Upset of the Year. In 2004, having vacated the WBO title, Sanders faced Wladimir's brother Vitali Klitschko in an unsuccessful challenge for the vacant World Boxing Council (WBC) and Ring heavyweight titles.

Nicknamed "The Sniper", Sanders was a southpaw with a long reach and was known for carrying formidable knockout power in his straight left hand.

Sanders vacated the WBO title in December 2003 so that he could concentrate on a challenge for the vacant WBC belt. The fight took place on 24 April 2004, against Wladimir's older brother, Vitali. Sanders was stopped by Vitali in the eighth round of the fight in the Staples Center in Los Angeles. After Sanders death, Vitali went on to pay his respects to Sanders by calling this the hardest fight of his career.