Alexander Povetkin viciously knocked out David Price with a left hook in Cardiff on Saturday.
The Russian remains the mandatory challenger to Anthony Joshua's WBA heavyweight title after ruining the hopes of popular Price in the fifth round.
Liverpool's Price briefly appeared on the verge of a fairy-tale result when he clambered off the canvas then came close to finishing Povetkin.
But a brutal combination of an overhand right followed by a left hook from Povetkin sealed a dramatic victory.
An overhand right from Povetkin had whirled passed its target inside the first minute and Price, with his own right hand, landed the first notable blow.
Price's confidence grew when he withstood a pair of sharp shots from his visitor in the second round, and he then began to earn respect with his jab.
There were audible gasps when Price hit the deck in the third, after his defence allowed a left hook through, but he rose to earn his own knock-down when Povetkin was ploughed into the ropes.
But Povetkin had the final say by stiffening Price's legs with a right hand, then closing the show with a follow-up.
Price had previously suffered four stoppage defeats (although one, against Erkan Teper, was overturned into a no-contest) and hoped to rejuvenate his career as the underdog in Cardiff.
Povetkin's sole career loss in 35 fights was against Wladimir Klitschko five years ago - he is now a leading contender to challenge Joshua, although Dillian Whyte expressed his interest in fighting the Russian in June.