Even the WBA questioned the matchup, withdrawing its sanction for Golovkin’s title because Brook had never competed at middleweight. The decision only reinforced the widespread belief that Brook was taking on too much against one of boxing’s most devastating punchers.

Golovkin came out aggressively and quickly showed the size and power advantages everyone expected, buckling Brook’s legs in the opening round. But rather than fold, Brook responded with sharp combinations, accurate counters and excellent timing that repeatedly caught the champion clean. His speed and punch selection frustrated Golovkin more than many naturally bigger middleweights had managed.

Brook’s finest work came in the second round, when he enjoyed sustained success before disaster struck. During one exchange, Golovkin fractured Brook’s right eye socket.

Brook fought on despite the injury, but the broken eye socket steadily changed the fight. As his vision deteriorated, Golovkin found the target more consistently and began breaking him down.

Brook’s damaged vision made it increasingly difficult to avoid the incoming shots, and Golovkin steadily broke him down with heavy punches from both hands. Still, Brook continued firing back whenever opportunities presented themselves.

Golovkin later admitted Brook’s movement and timing forced him to abandon his preferred approach.

“I promised to bring ‘Big Drama Show,’ like street fight,” Golovkin said after the victory. “I don’t feel his power. I feel his distance. He has great distance. He feels [my power], and after the second round I understand that it’s not boxing. I need street fight. Just broke him. That’s it.”

Trainer Abel Sanchez also revealed afterward that he was unhappy with Golovkin’s approach.

“He was trying too hard to knock Kell out,” Sanchez said. “I was trying to tell him this is a 12-round fight. Just beat on him, beat on him. I wanted him to use the jab more.”

With Brook absorbing too many clean shots and his right eye badly damaged, trainer Dominic Ingle threw in the towel in the fifth round to save his fighter from further punishment. Remarkably, Brook was still standing when the fight was stopped, and one judge actually had him leading 39-37, while the other two scorecards were even at 38-38.

Brook left the ring with his reputation enhanced despite suffering the first defeat of his career. Many praised his willingness to challenge one of boxing’s most feared champions when several established middleweights had avoided Golovkin.

That bravery, however, came at a heavy cost. The fractured right eye socket proved to be a career-altering injury. Eight months later, Brook suffered another orbital fracture. This time to his left eye in his welterweight title loss to Errol Spence Jr. Although he continued boxing for several years, the former unbeaten champion was never quite the same fighter after the damage sustained in the Golovkin fight, making one of boxing’s boldest gambles one of its most costly as well.