What we learned from Gennady Golovkin vs. Kell Brook

Kell Brook suffered defeat for the first time in his professional career when he was stopped in the fifth round against Gennady Golovkin -- here is what we learned from their pulsating bout at London's O2 Arena:

Golovkin is good at bashing Brits

Golovkin has got the taste for British blood -- and wants more of it. This was the US-based Kazakh's UK debut, but his third win over a Briton. Matthew Macklin was left writhing in agony from a body shot delivered by Golovkin in 2013 and two years later body shots accounted for Martin Murray, who took the world champion the longest he has been in his career before being stopped in the 11th round.

Brook lasted five rounds and ended up in hospital. Golovkin liked it so much in London that he says he wants to return to the UK to face Billy Joe Saunders, the WBO champion. Saunders, from Hatfield, holds the only world middleweight title belt not in Golovkin's hands and he will want to secure a good pay day for what certainly be a tough night.

"I'm a boxer, so now I want a unification fight," Golovkin said after dealing with Brook. "First of all Billy Joe Saunders, and then Canelo. My goal is to hold all the belts in the middleweight division. I'm a boxer and I'm open for everybody. I would absolutely come back to the UK to fight. I love the UK, I'm a big fan of London."

Golovkin is his own worst problem

Who will want to fight the Kazakh now after his latest devastating performance? Saul 'Canelo' Alvarez did not fancy it and dropped a weight division, relinquishing his WBC middleweight title, to avoid a Sept. 17 showdown with GGG. Canelo says he now wants the fight in a year's time, but on this display Golovkin is showing no signs of erosion of age. By fighting a world champion two weights below him, Golovkin showed he is running out of opponents.

He wants Saunders next, but Saunders declined an offer to fight him at the start of the year. Who else is there? Perhaps a mandatory against Daniel Jacobs before a fight with Saunders, if he agrees to it. "If we can't make the Daniel Jacobs fight then maybe we do a unification fight," Tom Loeffler, Golovkin's promoter, told reporters. "We tried to make that [Saunders] fight before in the UK and it didn't go very far. Maybe they will see the reaction and there will be more of an incentive to get in the ring. Gennady is open to fight anyone. The WBA is going to order a mandatory with Danny Jacobs, so we will see how that goes. Hopefully we can make it."

Britain's James DeGale has a good shout of being regarded as the world No.1 at super-middleweight and was also mentioned as possible future opponent. "James DeGale would be a big fight over here or Gilberto Ramirez also at the weight above," added Loeffler. But jumping two weight divisions to face the winner of Sergey Kovalev-Andre Ward would be a challenge, even for Golovkin, whose critics will point to the lack of names on his record.

There was nothing wrong with the champ

Rumours pre-fight were that Golovkin was ill after arriving in London last Sunday. Apparently, he had a sore throat and Brook's promoter Eddie Hearn thought he looked drawn at the weigh-in. But Golovkin quickly proved there was nothing in the rumours and hurt Brook with a left hook and a body shot in the opening round. At 34, Golovkin looks at his peak and his punches are as damaging as ever after leaving Brook needing to go to hospital to be treated for what turned out to be a fractured eye socket. Brook tweeted he needs an operation this week and his injuries are testament to the destructive power of Golovkin. Loeffler said the rumours happened because Golovkin did not do any interviews after the Thursday head-to-head press conference and was then grumpy at the weigh-in after being caught in London traffic.

Brook retains his credibility, but not his unbeaten record

The Sheffield boxer suffered his first defeat in 37 fights and brutally discovered that Golovkin was a level above anything he had previously faced. But Brook showed courage in taking the fight against Golovkin when others at middleweight did not. Brook also showed courage in the ring too when he battled on with a badly damaged eye from the second round.

It was his corner who saved him from what appeared to be an inevitable KO. By throwing in the towel, trainer Dominic Ingle also preserved Brook's longevity in the game and ensured he was not left permanently damaged by a battering. "I'm very proud of Kell Brook and he has big future still in boxing," said promoter Eddie Hearn of Brook, who landed some shots flush on the champion in the second and fourth rounds.

Brook is no middleweight, and maybe not a welterweight either

Abel Sanchez, GGG's trainer, delivered the best line of the head-to-head press conference on Thursday when he said: "Kell Brook will discover it takes more than a ten week scientific experiment to beat Gennady Golovkin". It was as devastating as one of Golovkin's body shots and Sanchez was proved correct. Just as Brook's fellow Briton and bitter rival Amir Khan discovered when he jumped two weight divisions in May to fight Saul 'Canelo' Alvarez, it is hard to make a seamless adjustment to middleweight on your debut in the weight class.

Brook -- still the IBF world welterweight champion -- said in the ring he will now consider operating as a light-middleweight and wants the winner of next Saturday's fight between Alvarez and Liverpool's WBO champion Liam Smith. "I have put a lot of muscle on and I think I'd be more suited to light-middleweight," he said.