ANB ARENA, SAUDI ARABIA -- There's life in the old dog yet. Callum Smith's bruising body work and a ruthless right hand that couldn't miss helped him inflict Joshua Buatsi's first career loss, snatching the WBO interim light-heavyweight world title with a unanimous decision nod after 12 gruelling rounds in an absorbing all-British clash.

There was a sound of utter disbelief echoing around the arena after a 119-110 scorecard came, though 115-113 and 116-112 scores were fairer for Smith (31-2, 22 KOs) as his defiance showed in a fine championship opener on DAZN pay-per-view.

Buatsi (19-1, 13 KOs) landed heavy shots but the 31-year-old's tendency to absorb too much damage ultimately proved decisive against the best boxer of his eight-year pro career, Smith weathering hairy moments as he also looked weary.

The Croydon man used his physicality early as they jostled for position, though Smith replied as you'd expect - landing body, then head shots before the interim champion fired back with the same before adding uppercuts for good measure.

After a brief rest period, a back-and-forth ensued as Smith dug to the body well and Buatsi again replied, the former losing his footing against the ropes before referee Howard Foster rightly ruled it a slip.

Buatsi started the second sharper, Smith stabbing to the body and the younger man was soon lectured for cuffing the Scouser's head down when chaining together combinations. They exchanged jabs but Buatsi did more damage in another competitive stanza.

Smith landed a three-punch combo, then Buatsi replied with one of his own midway through another tense round, both fighters' teams making their voices heard whenever their man enjoyed success. Work downstairs helped Buatsi briefly stagger the challenger onto his haunches, though Mundo finished the round with a straight right.

They continued to exchange in the pocket during a bruising fourth, Smith continued finding a home to both sides of Buatsi's body before the champion pinged his head back with uppercuts and straight shots - wearing the damage with a cut streaming down his right eye as smiles were exchanged through 12 minutes.

Could this pace be sustained for another eight? Into the fifth, Buatsi let his jab flow in the early going before unsuccessfully attempting to rough Smith up against the ropes.

Instead, they again exchanged on the inside but Buatsi continued walking forward even as Smith hurled away hooks and forced a temporary stoppage after knocking the champion's gumshield loose in the corner during another back-and-forth round.

Smith's best work came in this round, Buatsi as he often does, comfortable being tagged when the optics look far from comfortable from a judging perspective.

Badly wobbled and on unsteady legs for half the round as Smith's body work continued, followed by a whizzing right hand, Buatsi saved himself with a perfectly-timed counter left with referee looking intently, almost ready to step in.

Into round seven, Smith went back to what worked a treat - attacking downstairs, Buatsi far too open and seemingly at an energy deficit to do little more than return fire, unwise as it may have seemed.

They traded overhand rights in the final minute, crowd making their voices heard anticipating a knockout punch at any moment, though the pace slowed shortly afterwards and neither could really afford it, if this went the 12-round distance.

Early in the eighth, Smith landed a stinging left-right combo before again unloading an overhand right to ping Buatsi's head back.

The champion's success, behind the jab, wasn't enough to deter more of the same steady diet in the ninth, curling hooks and piercing the 31-year-old's guard as a crowd-pleasing right connected flush - Smith couldn't miss with that shot.

Still defensively susceptible to body work and counters, Buatsi had Smith looking weary during the bulk of another exhausting round in the tenth. Uppercuts and cuffing shots to the body worked a treat but importantly, an inability to pin Smith in a place he couldn't escape from proved costly, as Smith finished better during a round he'd lost.

The fight hung in the balance with two rounds to go, Buatsi being ticked off for rabbit punching as he teed off on Smith as the 34-year-old was starting to whizz wide of the target now, his corner's urgency levels increasing. They would've sensed they needed to encourage their man, now cut and bloodied, as the final frame came and they were met with a warm round of applause.

Smith rocked Buatsi early in the round, one the champion needed to win, looking exhausted as he walked forward onto more shots before another untimely stoppage for the gumshield coming loose a second time. Buatsi's power was stifled, Smith returning fire and ultimately a clear winner in a round both needed to win.