Oleksandr Usyk of Ukraine retained his world heavyweight titles with a ninth-round knockout of Britain's Daniel Dubois in Wroclaw, Poland on Saturday.

Victory meant Usyk kept hold of the World Boxing Association, World Boxing Organisation and International Boxing Federation belts.

It also extended his perfect professional record to 21 wins from 21 bouts.

Dubois, however, claimed he had been "cheated" out of victory after referee Luis Pabon ruled he had struck Usyk with a low blow in the fifth round, which led to a lengthy delay.

But former World Boxing Council super-middleweight champion Richie Woodhall, commentating at ringside, described his fellow Briton's punch as a "borderline shot".

Usyk recovered to drop Dubois down on one knee in the eighth round after landing with several punches to the head.

Dubois just beat the count before the bell sounded for the end of the round.

But the fight was all over in the next, with Usyk sending Dubois onto the canvas.

This time the challenger was unable to get back to his feet before the referee counted to 10.

"I feel good. I am grateful for my team, my family, my children. I love you," Usyk told TNT Sports.

"I'm grateful for my country and the Ukrainian army. Thank you so much."

Asked about the prospect of a unification bout with Britain's Tyson Fury, the World Boxing Council heavyweight champion, Usyk replied: "I'm ready tomorrow".

Dubois, however, was adamant his punch in the fifth round was legitimate, saying: "I didn't think that was a low blow, I thought it landed and that I've been cheated out of it tonight. I'll come again."

- 'Rematch' -

Veteran promoter Frank Warren added: "It wasn't a low blow. I wish they could put it up on there now and we could all see it. They didn't take any points off him.

"I like Usyk, but that was a complete 'home' decision. He was not fit to go on and they gave him a couple of minutes to recover... The governing bodies will have to look at that and order a rematch."

The 36-year-old Usyk was roared on by a large contingent of Ukrainian fans, with many of his compatriots now living in Poland following Russia's invasion of their country last year.

Ukrainian President Volodymyr Zelensky appeared on the giant screens before the fight to deliver a patriotic rallying cry amid the ongoing war with Russia.

Usyk had the better of the early exchanges but rank outsider Dubois, a 25-year-old Londoner, defied predictions he'd suffer an early defeat.

Then came Dubois's controversial punch in the fifth, which may have been on the belt line in some eyes.

After the bout resumed, Dubois attacked and traded punches with Usyk.

The ill-feeling between the duo was visible at the end of the round and Dubois continued his attack to the body in the sixth.

But Usyk started to dominate from then on, dropping Dubois in the eighth round before charging in again in the ninth, flooring the challenger once more before Pabon waved the fight off.