Undefeated American Adrien Broner stopped Britain's Gavin Rees in the fifth round on Saturday to retain his World Boxing Council lightweight world crown.

Broner dropped the 32-year-old Welshman to the canvas with right and left uppercuts and landed a ferocious flurry before referee Earl Brown stopped the fight after two minutes and 59 seconds of the fifth round.

"I'm fresh. I'm fly. I'm flashy. I did my job," Broner said. "If they didn't stop the fight he was going to sleep anyway. Rock-a-bye baby."

With a successful first defense of the crown he won last November by stopping Mexican southpaw Antonio DeMarco in the eighth round, the 23-year-old American improved to 26-0 with his 22nd stoppage inside the distance.

Rees fell to 37-2 with one drawn. The former World Boxing Association light welterweight champion gave away size and reach advantages, but pressed the attack early and after being flattened twice.

"That was a tough steak but I ate it," Broner said. "I knew he was going to come to fight. Tough world-class fighter. I had to see how much gas was in that little Toyota."

Broner landed a hard right uppercut to the chin of Rees after 52 seconds of the fourth round, sending the Welshman to the canvas for only the second time in his career.

A left uppercut to the body by Broner 30 seconds before the end of the fifth round dropped Rees to one knee. The challenger rose and exchanged again but Broner unleashed a final flurry and Brown had seen enough.

Rees pressed the attack early, landing body blows to the champion steadily in the first two rounds and nailing Broner with a left hook to the jaw.

But Broner seized command in the third round, landing a hard left hook and a punishing lead right that launched an eight-punch flurry before Rees, backed up against the ropes, could grab the champion.

On the undercard, Cameroon-born Australian Sakio Bika downed Montenegro's previously undefeated Nikola Sjekloca by unanimous decision to become the mandatory challenger for unbeaten WBC super-middleweight champion Andre Ward.

Bika won by judges' scores of 118-112, 119-109 and 120-108 to improve his record to 31-5 with two drawn while his European rival fell to 25-1.