Australian Tim Tszyu downs Tony Harrison in Sydney, next stop Jermell Charlo

Tszyu, the son of former undisputed light-welterweight world champion Kostya, sent Harrison to the canvas with a volley of punches. Although his opponent regained his feet to beat the count, the referee stopped the fight.

The Australian, who improves his record to 22-0 with 16 knockouts, was supposed to fight Charlo in January before the veteran American, who owns the WBO, WBC, IBF and WBA light middleweight belts, broke his left hand.

Tszyu instead lined up Harrison, a former WBC champion and the only boxer to have beaten Charlo, and on Sunday burnished his credentials with an impressive victory.

"I just beat the man who beat the man. What does that make me?" Tszyu said in the ring. "The message was sent clearly. You know what's up. You know what's next. I'm coming."

The Australian was always on the front foot against the experienced Detroit native, who relied on his razor-sharp jab and ringcraft to keep the younger fighter at bay.

In the third, Tszyu landed a big right hand on the counter that rocked Harrison onto the ropes but the American survived until the end of the round and worked the inside to get himself back into the fight.

Another good combination in the seventh round stunned Harrison but it was not until two rounds later that Tszyu was able to finish the fight.

Harrison was already on the ropes when Tszyu landed a savage flurry of six uppercuts before a right-left-right combination sent the American slumping to the floor amid the roars of the partisan crowd.

"The better man won tonight," said Harrison (29-4-1). "Tim, congratulations my boy."

Charlo appeared on the TV broadcast on Sunday and said he would be ready to fight by the northern hemisphere summer.

"He's going to be a tough fight because he's coming forward, but I think he's perfect for my style," Charlo said.

"I know he can't take my punch because I just know my punch is different. It just made me want to fight, just get this over with, shut him up. I've shut a lot of them up, it's just another one."