LONDON -- As unified middleweight titleholder Gennady Golovkin prepares to make his 17th defense, against England's Kell Brook, there's a third wheel muscling in on the attention being paid to their fight: lineal middleweight champion Canelo Alvarez.

Gennady Golovkin-Kell Brook Where: O2 Arena in London, England

When: Saturday

TV: HBO, 5:30 p.m. ET/PT

Golovkin, a knockout machine and perhaps boxing's most feared fighter, has come to the birthplace of boxing to defend against Brook on his turf on Saturday (HBO, 5:30 p.m. ET/PT with a replay at 10 p.m. ET/PT) at the sold-out O2 Arena because Alvarez has steadfastly refused to fight him, going so far as to relinquish his sanctioning organization belt rather than fulfill a mandatory obligation to Golovkin in a megafight.

The typically mild-mannered Golovkin normally doesn't utter a negative word about anybody, but Alvarez's gamesmanship clearly has annoyed him. He has been more outspoken about the situation than ever before.

"It's like a circus, too many clowns," Golovkin said. "Canelo knows he doesn't want to fight me. Why do you keep talking then?"

Although Alvarez won the lineal middleweight title, he did so at his preferred catch weight of 155 pounds against Miguel Cotto last November and continues to resist a move to 160 pounds. Instead of agreeing to face Golovkin, as he appeared to do following his huge knockout of Amir Khan, a welterweight who moved up, in a May defense, Alvarez is retreating to junior middleweight. He will return to 154 pounds, where he once held titles, to challenge titleholder Liam Smith, Brook's countryman, on Sept. 17 (HBO PPV) at AT&T Stadium in Arlington, Texas.

Golovkin, who has knocked out 22 consecutive opponents since 2008 and owns the best knockout percentage in middleweight title history (.914), is resistant to any criticism that he is picking on a welterweight in Brook, who owns a world title and is considered by many No. 1 at 147 pounds, the same way Alvarez did with Khan.

"Canelo was the middleweight champion and chose Amir Khan," said Golovkin, who has been rebuffed by a who's who of the middleweight division in recent years. "We didn't choose Brook. He chose me. My goal is all the middleweight belts. Canelo is not ready until next year or maybe not for five years."

Gennady Golovkin, left, defeated Dominic Wade in April to extend his KO streak to 22 in a row. Harry How/Getty Images

The 30-year-old Brook (36-0, 25 KOs) had no fear of facing Golovkin (35-0, 32 KOs), jumping at the chance when offered the fight after a bout between GGG and another Brit, middleweight contender Chris Eubank Jr., fell apart in the final moments when Eubank allowed the deadline to pass without signing.

"This fight is much bigger than if I faced Eubank," Golovkin said. "[Brook] is a champion, he's undefeated, he is more famous. Who knows Eubank? Everybody understands he is 50 percent clown, 50 percent fighter. OK, he says he's ready, he's fighting. [Then] he's not ready, more dancing, more talking, trash talking. He's like clown."

Brook is moving up two weight divisions in an attempt to become the third reigning welterweight titlist to win a middleweight title in boxing history, joining all-time greats Sugar Ray Robinson and Carmen Basilio. Kazakhstan's Golovkin, 34, who fights out of Santa Monica, California, respects the fire Brook has shown in accepting this difficult assignment.

"He is not ready. I told him, 'Please just give me my belt,' and that's it. He says OK and steps back. Too much talking, too much trash talking. Just give me my belt and get out of here. Maybe it's business. After [the Khan] press conference I lost respect for him because he talks too much." Gennady Golovkin on Canelo Alavrez

"Everybody has risk in boxing," Golovkin said. "He's moving up and taking that risk, and for that I have a great deal of respect for him. Brook is dangerous. He has a very high boxing IQ and we're fighting in his country."

Golovkin, however, said he has little respect for Alvarez after the way he led him to believe they would fight on next week's Mexican Independence Day weekend.

After Alvarez knocked out Khan, he called Golovkin into the ring, telling him to his face that "Mexicans don't f--- around" and seemingly promising they would fight next.

At the postfight news conference, Alvarez said he would even fight GGG at 160 pounds, and his promoter, Golden Boy's Oscar De La Hoya, put on an over-the-top display, telling the media that Golovkin promoter Tom Loeffler, of K2 Promotions, should make sure his phone was on because he would be calling him the following day to open discussions.

Welterweight titlist Kell Brook has been waiting for a big fight at welterweight for a long time. Adrian Dennis/Getty Images

But instead of trying to make the fight, Alvarez vacated his title, which interim titlist Golovkin inherited, and eventually signed to fight Smith, who was all but unknown outside the United Kingdom. Alvarez also promised that he would fight Golovkin next September, although there is no formal agreement and few believe him.

Golovkin does not think it will happen and that Alvarez will continue to make him and the public wait for the big fight.

"Impossible," he said of Alvarez agreeing to fight him in 2017. "But maybe if I look bad with Kell Brook or another guy and he's ready, but [only] if I look bad."

"Kell Brook has my full respect. It's not easy for us to get opponents. He's ready for me and I'm ready for him. Kell Brook is the biggest test to me." Gennady Golovkin

An Alvarez-Golovkin fight looms as one of the biggest in boxing, but it could play out like the frustrating Floyd Mayweather-Manny Pacquiao fight, which took years to make. Golden Boy's Eric Gomez, promoted this week to president of the company, has said repeatedly to ESPN.com that Alvarez will fight Golovkin, but he would not give a timetable.

"That is the biggest fight, with Canelo," Golovkin said. "But he stepped back. He is like loser. After Khan he said, 'OK, G, come into the ring.' I'm asking, 'What you want?' He said, 'I'm OK, I looked good. We fight.' Then at the press conference, he says, 'I am ready, give me gloves. I'm ready right now.' Too much talking. This is not true. He is saying, 'I am ready [but] maybe not next fight. Next year.' Everybody understands he said I am ready right now. I gave him too much attention. I do not believe he is ready.

"I know him. We trained together, like, three training camps. We sparred together. I know his team. He is not ready. I told him, 'Please just give me my belt,' and that's it. He says OK and steps back. Too much talking, too much trash talking. Just give me my belt and get out of here. Maybe it's business. After [the Khan] press conference I lost respect for him because he talks too much."

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Abel Sanchez, Golovkin's trainer and the 2015 Boxing Writers Association trainer of the year, said Golovkin is angry that Alvarez is playing games.

"When the whole Canelo fiasco first started, when he fought Cotto, we were trying to just be laid back and let him talk and let him do his thing," Sanchez said. "Once he called Golovkin into the ring, Golovkin was very offended. He may not say that to you, but I know Golovkin and Golovkin was like, 'What the hell did they call me in here for if he's not going to be true to his word?'

"But when he did that and he showed all that bravado and then backed down, I think it set off something in Golovkin that was like, 'You know what? I'm through being a nice guy. I'm through not saying the things that I honestly think.'"

Sanchez shares the opinion of Golovkin and many others who do not believe Alvarez will fight him next year.

"I don't think so. We've had that discussion, and I honestly don't think so either," Sanchez said. "I think Canelo knows he doesn't stand a chance and I think that Oscar knows that he doesn't stand a chance. But the more they talk, the more they stay relevant. We as the GGG team just keep GGG busy. When you keep him busy, eventually the fans will have so much pressure on Canelo, as the Mexican fans are doing right now, and the journalists. Don't talk if you're not going to back it up."

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Sanchez said that had Alvarez and Golden Boy simply told Golovkin that Alvarez was not ready for the fight, vacated and said he would come back and fight Golovkin for the title in a few fights, Golovkin would have been cool with that.

"But he called G in the ring and said some bad things in Spanish and then Oscar goes in the press conference and says, 'Tom, make sure you answer the phone.' Come on," Sanchez said.

Then along came Brook, daring to be great and going up to fight the man nobody else wants to face. Golovkin and Sanchez both respect that. A lot.

"I think a guy like Kell and a guy like [Sugar Ray] Leonard and guys like Robinson, those guys are wanting to test themselves to see where they're at," Sanchez said. "There's nothing wrong with saying, 'You know what? That guy was just too big [if you lose].' But when they introduce you as middleweight champion and you talk all kinds of nonsense and then give up the title, how can you have any respect for somebody like that?"

Said Golovkin: "Kell Brook has my full respect. It's not easy for us to get opponents. He's ready for me and I'm ready for him. Kell Brook is the biggest test to me. He's a great champion, he's moving up in weight, but great fighters have done that and I know he's a great boxer and he will test me the most."