Unified middleweight champion Gennady Golovkin thanked Kell Brook for indulging his desire for a crazy street fight at London's O2 Arena.

Golovkin, who now has 33 stoppage wins in an unblemished 36-fight record, handed career welterweight Brook his first professional loss over five brutal and thrilling rounds on the banks of the Thames.

The 34-year-old Kazakh superstar had his WBA and IBF belts on the line and hurt Brook with a shuddering left hook in the opening session before the 147-pound IBF ruler sensationally elected to fight fire with fire.

Ultimately, Golovkin's fearsome punching power and suffocating ring generalship sapped Brook's resistance and the challenger's face was battered and bloodied when his corner threw in the towel after one minute and 57 seconds of the fifth.

Brook was unable to attend the post-fight news conference as he went to hospital with a suspected broken right eye socket, leaving Golovkin to hold court – marvelling at his vanquished boxing foe's skill and bravery but stating the jump in weight was too big a gap for him to bridge.

"I don't feel power. I feel very good speed and distance. Kell, he is a very good fighter," said Golovkin, who himself wore the scars of battle after Brook scored frequently enough with precision hooks and uppercuts in centre ring.

"After the first round I feel like I was stronger a little bit. In the second round… I wanted not boxing.

"I'm crazy, I want a street fight as well as it being a dramatic show.

"I believe, I feel he is a very good fighter, but he is not middleweight division. Maybe he is strong, he looks good but not [against me in his] first [middleweight] fight. He needs time. I respect Kell and his team.

"I know I took too much punches [but] I don't feel power, so I was like, 'come on, come on, come on I want more!' It's crazy."

Dominic Ingle, Brook's long-serving trainer, drew the ire of a crowd engrossed in the spectacle for halting the action but Golovkin was in no doubt that it was the right call having seen his opponent flagging under his wrecking-ball fists.

"I think this was smart. I respect his corner and his coach. Seriously, in the last round he feels very bad," he added.

"He can't breathe, he can't move and he's finished. It's smart and I respect his corner."