The boxing word of the day: Anticlimax.

British welterweight Kell Brook rose two weightclasses to give 160-pound champ Gennady Golovkin the sternest competitive test of his title reign, but had it abruptly cut short in the fifth round when his trainer tossed in the towel as a surrender after a prolonged flurry along the ropes.

The sudden win, before a packed house at the O2 Arena in London, allowed Golovkin to defend his International Boxing Organization middleweight belt for the 14th time since 2011 -- along with the IBF and WBC straps he'd subsequently captured.

He's also the WBA champion in the weight class, but that organization declined to sanction the bout, citing Brook's lack of experience as a middleweight.

CBS Sports had scored the fight even -- two rounds apiece -- when it ended.

The Brook-favoring crowd wasn't happy with the stoppage of what had been a compelling fight, but both Golovkin and his trainer, Abel Sanchez, claimed a decisive end was near anyway.

"Kell is good fighter. But he is not middleweight," said Golovkin, who rated his performance a "3 or 4" on a scale from 1 to 10. "I wanted to bring a big drama show like a street fight. I feel like it was finished, it was over. Maybe a couple rounds more and that's it. I didn't feel any power."

Sanchez, who told CBS Sports before the fight that no opponent from 154 to 168 pounds could last the distance with his man -- who's now knocked out 23 in a row since 2008 -- agreed with Golovkin's assessment.

"He was trying too hard to knock Kell out," Sanchez said. "I told him to 'just beat on him, beat on him, practice.' The corner did the right thing. At that point when they stopped it, it was over. Eventually he was going to hurt him permanently."

Brook claimed he sustained a right eye injury in the early second round and he did have a noticeable red mark underneath it. He pointed to the area and rubbed it several times during the action.

Upon recognizing that his corner had tossed the towel, Brook immediately turned toward trainer Dominic Ingle with his arms out, and Ingle gestured toward the eye as if preserving the fighter's health had been the catalyst.

Still, Brook said he was disappointed with the decision.

"Absolutely (I could have kept fighting). It should have carried on," he said. "Dom has seen me since I was 9 years old, so I don't know what he's seeing from the outside, but I wanted to carry on. If you're going to beat me, knock me out. I'm a competitor. I'm very frustrated."

Unbeaten in his initial 36 pro outings, Brook entered as a 6-to-1 underdog but held his own throughout the first 12 minutes of action.

He was wobbled by a Golovkin left hook midway through the first round, but was able to move and avoid further punishment and actually finished the round with a scoring flurry. He returned to win the second round with a series of clean straight shots that snapped Golovkin's head back and were flashy, albeit not devastating.

Brook hit the floor early in the third while taking a left hook and retreating at the same time, though the exchange was ruled a slip rather than a knockdown. Golovkin continued to press throughout the three minutes and seemed intent on making it a rugged fight more than a skills contest. He landed the best shot of the fourth, too, in the form of a straight right hand along the ropes, but Brook won the round thanks to a series of exchanges across the three minutes.

Golovkin, though, was in control in the fifth and had landed heavy shots as Brook slid across the ropes. The challenger didn't appear significantly rattled or in danger of being knocked down, but he did look tired, which probably fueled Ingle's concession.

"We knew (Brook) was going to be fast but we knew that we'd break him down," Sanchez said. "He didn't look as good as we wanted him to, but we won."

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