I wanted to show Americans don't lay down Lamon Brewster

Klitschko was dominating the contest until he got caught midway through the fifth round with two left hooks.

Referee Robert Byrd stopped the fight after the round ended, ruling that Klitschko was unable to carry on.

"It was kill or be killed," said 9-1 underdog Brewster, who was fighting for the first time in 13 months.

Klitschko knocked Brewster to the canvas in the fourth round with a vicious right hand.

But Brewster got up, with both fighters falling over after tangling in the American's corner at the end of the round.

Midway through the fifth, Brewster landed the left hooks that changed the fight.

Klitschko just about survived the initial onslaught but was in trouble again when the bell sounded.

He went down again, forcing Byrd to intervene.

"He couldn't take care of himself," said Byrd. "I tried to get a response out of him but there was none. I've never stopped a fight like that before."

Klitschko's future is now in some doubt after he was knocked out for the second time in four fights.

Klitschko looks a beaten man

Corrie Sanders stopped him in Germany last year and will fight his brother, Vitali, in a fortnight's time.

"I know he's a tough guy. I know he can punch," said Brewster. "But what I wanted to do was show Americans don't lay down.

"I knew he would get tired. He'd either get tired of hitting me or get tired of me coming forward and putting pressure to his face."

Klitschko was taken to a hospital for a precautionary brain scan on the advice of ring doctor Margaret Goodman.