Artur Beterbiev was forced to battle through a horrendous cut to finally end Marcus Browne's hopes in a ninth-round stoppage in Montreal on Friday night.
The fearsome IBF and WBC light-heavyweight champion sustained the wound in the centre of his forehead after an accidental clash of heads in the fourth round.
His undefeated record was in jeopardy when, at the beginning of the fifth, the referee told him he may only be allowed one more round due to the cut.
But Beterbiev took over the fight and knocked his challenger down twice, eventually stopping him in the ninth.
Beterbiev maintained his stunning record as the only world champion in the entire sport with a 100 per cent knockout record in his 17th fight.
His reign was rescued by his ability to increase the tempo immediately after the referee's threat to end the fight in the fifth round.
By this time Browne was also cut under Beterbiev's onslaught.
Browne narrowly avoiding crumpling to the floor in the final exchange of the sixth round.
Beterbiev, with blood pouring down his face, was hunting for the stoppage and sent Browne to the canvas in the seventh round after a body shot and a right hand.
A left uppercut in the ninth round sent a battered Browne to the floor again, and he was unable to beat the referee's count.
Beterbiev remains the unified light-heavyweight champion and one of boxing's hardest-hitting fighters.
His rival world champions in the light-heavyweight division are Dmitry Bivol (WBA) and Joe Smith Jr (WBO).
Sky Sports Boxing schedule
Saturday December 18 - Top Rank in Montreal
Artur Beterbiev vs Marcus Browne - IBF and WBC light-heavyweight titles
Saturday January 29 - BOXXER in Cardiff
Chris Eubank Jr vs Liam Williams
Claressa Shields vs Ema Kozin - IBF, WBA and WBC middleweight titles
Saturday February 19 - BOXXER in Manchester
Amir Khan vs Kell Brook
Saturday February 26 - Top Rank in Glasgow
Josh Taylor vs Jack Catterall - undisputed super-lightweight titles