Vitali Klitschko retained his WBC heavyweight title by knocking out Cuba`s former Olympic champion Odlanier Solis in the first round in Cologne, according to BBC.
The Ukrainian, 39, caught his opponent with a decent shot towards the end of the round and Solis suffered a serious right knee injury as he went down.
Klitschko was initially furious that Solis was not carrying on and 19,000 watching fans also voiced their anger.
But the referee ruled that the Cuban was unable to continue the fight.
A diagnosis at Cologne`s University Hospital revealed Solis tore his cruciate ligament and damaged cartilage in the fall, while further tests on Sunday will reveal whether he needs surgery.
"I do not think there will be any other outcome than Solis will have to take a long break to recover," said his promoter Ahmet Oener after the WBC`s top-ranked challenger limped out of hospital on crutches.
According to German newspaper Die Welt, Solis had already had a problem with his right knee before the fight.
"We knew it, but we didn`t want to let it jeopardise the fight against Vitali," said the Cologne Arena spokesman Malte Mueller-Michaelis.
"His manager Jose Perez knew about the previous problem, but it was thought it would go away if there was enough training and the muscle stabilised the knee."
Solis, 30, went into the ring with a perfect 17-0 pro record (12 by KO), and began confidently enough.
Both men were still getting the measure of each other when Klitschko landed a solid right on the side of Solis`s head.
The punch almost seemed to have a delayed reaction as Solis crumpled backwards, and he was initially unable to rise, clutching his knee.
"It was not easy at the start. Solis was pretty fast but he got a hard punch from me," Klitschko said in a ringside interview. "I am so sorry for everybody who wanted a longer fight."
A dejected Solis stated: "I hurt my knee when I went down. It was unlucky but unfortunately these things happen. It will take a lot of time for me to get over this because I am not used to losing."
With younger brother Wladimir, the IBF and WBO heavyweight champion in Vitali`s corner, the fight was watched at ringside by Adam Booth, the trainer-manager of WBA title holder David Haye.
Britain`s Haye is set to fight Wladimir in a unification bout this summer, and could also face Vitali at some stage.
But after the latest win for Vitali (42-2, 39 KOs), Wladimir said that Haye would be relieved to be facing him and not his older sibling.
"You can see why David Haye chose to fight me and not my brother. He was incredible today," said Wladimir.
"David is definitely going to say to Vitaly `I will fight you down the road but Wladimir is next because he is weaker`."