LAS VEGAS — Gennady Golovkin sat on the sidewalk, about 50 feet from the rim, and nonchalantly tossed a basketball toward the net using only his upper body — swish.

Sure, he was simply goofing around, but it was another impressive feat of strength for the unified middleweight champion. He’s now 36, but his conditioning remains at the top of the sport.

After scoring 23 consecutive knockouts, Golovkin (38-0-1, 34 knockouts) eked out a decision over Daniel Jacobs and was held to a controversial draw against Canelo Alvarez last year. There are some critics who claim that being taken the 12-round distance in back-to-back fights is proof that GGG is showing his age. Really, they were the first two elite opponents of Golovkin’s career after years of being avoided by the best middleweights.

Golovkin proved his motor is as high-powered as ever with a steady work rate in both bouts, and when he returned in May with a second-round destruction of Vanes Martirosyan, it was nine punches strung together that did the trick.

As he prepares for fight No. 40 with a man eight years his junior, the highly anticipated rematch with Canelo on Saturday at T-Mobile Arena (8 p.m., HBO PPV), GGG shows no signs of slowing down at an age where many of his contemporaries are shopworn.

“It’s only a select few … those people who are professional about their business and professional about their conduct inside and outside the ring” who can fight at the elite level at an advanced age, Golovkin’s trainer, Abel Sanchez, says to The Ring. “For Golovkin, I feel it’s the Kazakh (amateur) system that prepared him for the rigors of Abel Sanchez training.

“I think it’s how he grew up. They make sure their fighters are not only technically fit but also physically fit and they teach them how to work hard.”

GGG, who now resides in Santa Monica, California, shuns alcohol and his workouts are the stuff of legends. He routinely pumps out 4,500 sit-ups a day during training camp in addition to his lengthy runs in the mountains of Big Big Bear Lake, California.

Top heavyweight prospect Joe Joyce, who won a silver medal at the 2016 Olympics, recently joined Sanchez’s stable and marvels at the way Golovkin pushes himself in camp.

Boxing is a lonely sport, but it’s a team atmosphere at Big Bear and GGG (also an Olympic silver medalist) is the de facto captain of the squad. The other fighters — like Joyce, former titleholder Alfredo Angulo and junior welterweight contender Ryan Martin — follow Golovkin’s lead through the arduous sprints through the residential streets of Big Bear, weaving their way through the orange cones.

Golovkin clearly enjoys setting the tempo; displaying the work ethic necessary to ascend to the upper echelon of an unforgiving sport. “Not genetics,” says The Ring’s No. 1 pound-for-pound fighter when asked what he owes his longevity to. “I think it’s healthy life.”

Gennady Golovkin and Abel Sanchez. Photo / Tom Hogan-GGG Promotions

When Golovkin first linked up with Sanchez in 2010, he was immediately drawn to the solitude — and altitude — of the mountains of Big Bear. It’s Golovkin’s laser focus in camp that is often credited with his incredible success. “Only boxing,” Golovkin often says.

He loved that he could train whenever he wanted while living in a cabin at Big Bear, and he’s known to sneak in a night session every now and then — the gym is never closed. It’s the kind of commitment to the sport echoed by fighters such as Floyd Mayweather Jr., Bernard Hopkins and Wladimir Klitschko, other men competed at the elite level into their 40s. Sanchez sees no reason why Golovkin can’t join them.

To that end, Sanchez has tapered down the number of rounds Golovkin spars as he ages. He’s thankful for the lessened toll on his body, but it’s really Golovkin’s mind that will dictate how long he can box near his best form.

“They have to have the challenges. It’s not about the physical challenge; it’s the mental challenge. Andre Ward retired because there were no more mental challenges for him. Physically, he could get ready for any fight, but when you have that perceived threat, it makes you go back in the gym.

“It’s not the fights; it’s the 10 weeks in the gym that wear your ass out. For Gennady, he’s been doing since he was 10 years old. … 26 years of the same thing over and over and over again. It’s monotonous.”

Assuming GGG tops Canelo clearly Saturday and there is no trilogy, there a few potential matchups that would interest Golovkin, according to Sanchez. One such fight would pit Golovkin against crafty southpaw Billy Joe Saunders in a unification title tilt, but the Brit first must defeat Demetrius Andrade in a coin-flip bout scheduled for October 20.

Another challenge: whoever emerges in the 160-pound title bout between Jacobs and Sergey Derevyanchenko on October 27 because “he has that vengeance to try get that title back” after Golovkin was stripped this summer. The key to create a true challenge in the competitor’s mind, Sanchez says, is public interest.

“We trained in the prior training camps hard obviously; always hard,” Sanchez says. “But this time, after (Canelo’s) positive test (for the banned substance clenbuterol) it was a different edge to GGG. He thought he was insulted; disrespected. The camp was more purposeful.

“He hasn’t been dominated yet. he’s undefeated … you can cite his age and say he’s erording, but until someone dominates him in the ring or in training, I can’t agree with that.”

GGG says it’s business, not personal, even after all the ill will he seems to harbor dating back to the judges’ decision in September and Alvarez’s positive drug test that cost Golovkin tens of millions of dollars.

Golovkin’s approaching to training is a throwback to an era where top-level fighters competed at least five times a year. He’s out to prove that he’s far from old, and that good ol’ fashioned hard work trumps youth.

“Old school is back,” Golovkin says.

Mike Coppinger is the Senior Writer for RingTV.com. Follow him on Twitter: @MikeCoppinger