Abraham (left) is knocked down by Dirrell (right) in the fourth round
American Andre Dirrell beat German Arthur Abraham on a disqualification for a shock win in the Super Six super middleweight tournament in Detroit.
Dirrell slipped in the 11th round as he took a glancing blow to the chin from Abraham who then struck his opponent while he was kneeling on the canvas.
The foul resulted in Abraham's first loss in 31, although Dirrell was winning on points before the incident.
Abraham tops the six-man leaderboard by a point from Carl Froch and Andre Ward.
Dirrell's quick hands saw him dominate early on, and he floored Abraham in the fourth with a right.
"I was off-balance," Abraham said. "It wasn't a bad knock-down. I didn't have my balance."
Abraham rose to take a standing eight count, and he was in trouble again three rounds later when Dirrell opened a cut by Abraham's right eye.
IBF middleweight champion Abraham appeared to knock down Dirrell with a straight right in the 10th, but referee Laurence Cole ruled it was a slip.
When the German was disqualified in the 11th, Dirrell was ahead with all three judges - 97-92, 98-91, 97-92.
"He wasn't on the ground, I shouldn't be disqualified for this," Abraham protested afterwards.
Armenian-born Abraham came into the bout as the leader of the tournament with three points after he was knocked out American Jermain Taylor last October - Taylor subsequently quit the competition to be replaced by Allan Green.
Dirrell lost his opener to WBC super middleweight champion Froch on the same night in Nottingham.
WBA champion Ward matched unbeaten Froch in winning by decision but Abraham's extra point for a knockout gave him a lead after the first of three rounds of fights.
Dirrell next faces American compatriot Ward in his third and final group fight, with Abraham facing Froch later this year.
Froch's next bout, though, comes against the tournament favourite, Dane Mikkel Kessler in Herning, Denmark on 24 April.
The four leading points scorers will advance to the semi-finals, with the top-ranked facing the fourth, and second facing third.
The tournament, masterminded by American broadcaster Showtime, will culminate in early 2011 with the finalists battling it out for the inaugural Super Six World Boxing Classic trophy.
Meanwhile, in Las Vegas, Joan Guzman earned a 12-round split decision over Ali Funeka.
Dominican Guzman won on two cards - 114-113 and 116-111 - with the third judge going 114-113 in South African Funeka's favour.
Guzman, remains unbeaten, on 30-0 with one draw, but does not claim the vacant IBF lightweight belt after weighing in nine pounds over the 135-pound limit.