PHILADELPHIA — Russian world champion Artur Beterbiev unified the light heavyweight boxing division on Friday with a 10th-round stoppage of Oleksandr Gvozdyk, extending a remarkable knockout streak and undefeated pro career.

Beterbiev knocked his Ukrainian opponent down three times in the 10th before referee Gary Rosato stopped it with 11 seconds left.

"This moment, it is my most important fight," Beterbiev, 34, said in broken English. "I had big work to prepare for this fight."

Beterbiev, who represented Russia in the 2008 and 2012 Olympics, defended his International Boxing Federation crown and seized Gvozdyk's World Boxing Council belt.

Beterbiev, who trains in Montreal, Canada, improved to 15-0 with 15 knockouts and is the only major boxing champion to have knocked out all of his opponents.

Gvozdyk threw more punches, but Beterbiev landed the harder and more direct shots, mixing up his combinations by working the head and body at The Liacouras Center in Philadelphia.

Beterbiev showed in the final round why he is considered one of boxing's most devastating punchers.

Looking the fresher of the two, Beterbiev scored the first knockdown with a flicking left to the side of the face.

He knocked Gvozdyk down again 30 seconds later and finished the 2012 Olympic bronze medalist off with an overhand right.

Gvozdyk had been warned by Rosato after the second knockdown that the fight would be stopped if there was one more.

When Gvozdyk took a knee after absorbing the right, Rosato had no choice but to stop the bout.

Beterbiev was a slight underdog for the two-belt fight, which was the first time that two undefeated champions squared off in the light heavyweight division.

Gvozdyk, who suffered his first loss in 18 fights, was coming off a third-round knockout of Doudou Ngumbu.

This was the second time the pair had clashed, with Beterbiev also winning when they fought as amateurs 10 years ago.

Beterbiev's next fight will be a mandatory defence against another undefeated fighter, China's Meng Fanlong (16-0, 10 KOs).

Promoter Bob Arum wants to hold that fight early next year in China around the time of the Chinese New Year.

The fight was the first world title bout since American boxer Patrick Day died on Wednesday from brain injury suffered in a fight last weekend.

The 27-year-old super welterweight from New York had undergone emergency brain surgery after being knocked out by Charles Conwell last Saturday at Chicago's Wintrust Arena.

Day, who had been in a coma following the knockout, died surrounded by family and friends, his promoter said.

Cay had been carried unconscious from the ring on a stretcher after his 10th-round knockout.

He was honored at Friday's event with a 10-bell salute prior to the Beterbiev-Gvozdyk fight. — AFP