By Martyn Herman

LONDON (Reuters) - Middleweight marauder Gennady Golovkin retained his WBC and IBF titles when challenger Kell Brook was pulled out by his corner five rounds into a compelling fight on Saturday.

Kazakh Golovkin had won his previous 22 fights by knockout but was rocked by unbeaten welterweight world champion Brook in the early exchanges before his superior power told.

Brook, who suffered a suspected fractured eye socket, won the second round with a left uppercut that produced huge roars from the huge O2 Arena crowd, who were mainly supporting the British underdog.

He was still standing his ground in the fourth but by the fifth it became clear the step up from welterweight to fight the man who has brutally dominated the middleweight division for six years was a step too far.

There was little coming back as Golovkin landed some thumping blows and trainer Dominic Ingle threw in the towel which was initially unseen by the referee.

Some of the crowd booed, but afterwards Brook said the eye injury had left him with restricted vision.

Unbeaten Golovkin took his record to 36 wins.

"Kell is a huge fighter, a very good fighter, but he's not a middleweight. I respect him, he is good, but not so strong," 34-year-old Golovkin said at ringside.

"I respect Kell, he's a warrior. And his corner, for his career, for his family, was correct. It was game over."

Brook was wobbled by a huge body shot in the first round but connected with a couple of stinging jabs.

Golovkin has rarely been hurt in his outstanding career but he was marked around the face by the end of the second as the skilful Brook landed a fearsome uppercut.

The 34-year-old Golovkin looked rattled in the third as he missed with some sledgehammer shots, although he landed enough to leave Brook's right eye swollen.

The fourth round looked about even, with Golovkin relentlessly moving forward in search of the knockout and Brook countering with some precise jabs.

There was a menace about Golovkin in the fifth though and he began to unload in devastating fashion, forcing the intervention of Brook's corner as the gallant Sheffield fighter tasted defeat for the first time in his 37-fight pro career.

"He broke my eye socket in the second round. I was seeing three or four of him," Brook said.

"Believe me, I hurt him. I saw his legs start to buckle. I was starting to settle in. I would fight him again."

Golovkin, who remains the WBC, IBF and WBA champion, said his next target was a unification fight against another Briton, WBO titleholder Billy Joe Saunders.

The WBA had refused to sanction Saturday's fight fearing Brook, who has spent his career fighting men 13 pounds (6 kg) lighter than Golovkin, was putting his health in danger.

(Editing by Toby Davis/Peter Rutherford)