The fight was stopped with 56sec remaining in the round.
Klitschko improved his record to 45-2, while Charr, who was unbeaten before his meeting with Ukraine’s 41-year-old veteran, fell to 21-1.
“I was a bit disappointed as I wanted to win today by a real knockout, not a technical one,” said Klitschko.
“I was ready to continue the bout and Charr was also ready for further boxing but it was the decision of the doctor and the referee, who are responsible for it and we could do nothing about it.” Fighting in the 30,000-seater Olimpiyski indoor sports arena, the hugely-experienced Klitschko started the bout as a hot favourite in what was his 17th world heavyweight championship fight.
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