Britain's Anthony Joshua celebrates his victory over New Zealand's Joseph...

Britain's Anthony Joshua celebrates his victory over New Zealand's Joseph Parker, left, after their WBA, IBF, WBO and IBO Heavyweight Championship title bout in Cardiff, Wales, Saturday, March 31, 2018. Credit: AP / Frank Augstein

CARDIFF, Wales — After just 21 fights, Anthony Joshua is a three-belt world heavyweight boxing champion.

He finally knows what it’s like to go 12 rounds, too.

Frustrated by the movement of Joseph Parker and the irritating interventions of inexperienced referee Giuseppe Quartarone, Joshua required a decision by the judges to add the WBO belt to his WBA and IBF titles.

It was never in doubt, with two judges awarding it to Joshua 118-110 and the other making it 119-109. It was Parker’s first loss in 25 fights, and Joshua 21st straight win.

“I was switched on, focused and went 12 rounds. It was light work,” said Joshua, after a mature, patient but hardly headline-grabbing performance.

“This was about boxing finesse. I stuck to my word.”

All 20 of Joshua’s previous wins came via knockout — and only two had gone beyond seven rounds.

But, thanks to Parker’s reactions and counterattacking skills, one of the sport’s biggest punchers barely landed a clean shot with his big right hand.

Quartarone didn’t help, either, constantly getting between the two fighters and stopping the flow of the contest. There were boos at the final bell from many in the crowd of 78,000 inside Cardiff’s Principality Stadium, seemingly aimed at an Italian referee who was taking charge of his first world heavyweight title fight.

“Ref seemed too hurried to break them up at times,” tweeted former undisputed heavyweight champion Lennox Lewis. “But it was a clear win for AJ and he showed maturity (and) patience.”

Joshua moved to within one win of matching Lewis’ achievement of cleaning up the sport’s most glamorous division.

Big-punching American Deontay Wilder holds the other main heavyweight belt, the WBC’s, and could meet Joshua before the end of the 2018.

“Wilder, let’s go baby,” Joshua screamed in his interview inside the ring.

And he wants to stay in Britain to complete the set.

“I’m not interested in coming to America,” Joshua said. “We are staying right here.”

Neither Joshua nor Parker came close to getting knocked down, showing respect for each other’s undefeated records. They hugged after the final bell, though Parker knew he had been beaten.

“I was beaten by the better man,” said Parker, who demonstrated no nerves fighting in front of the biggest crowd of his career. “It’s been a good experience being here and I’m thankful for the opportunity.

“I want to go back and come back stronger.”

There is a rematch clause for the fight, but Joshua might have other ideas. Namely Wilder, or mandatory WBA challenger Alexander Povetkin, who beat David Price in the undercard.

After the epic nature of his 11th-round knockout of Wladimir Klitschko, Joshua has had to battle for two scrappy wins in Cardiff — first over the awkward Carlos Takam in October last year and now Parker.

Joshua controlled the center of the ring, using his superior reach and jab, but Parker — bobbing and swaying — proved a tough target.

One powerful left shook the WBO champion in the fourth, but Parker got through with a straight right through Joshua’s defense midway through the fifth, sending the Briton reeling for the first and only time.

Parker’s left eye was bleeding and starting to close by the end of the 10th round but he managed to take Joshua the distance. The result was emphatic, though.

It was only the second heavyweight unification title bout between unbeaten fighters, after Mike Tyson against Tony Tucker in 1987.

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