LAS VEGAS — Last Friday, Gennady Golovkin trained for the biggest moment of his life even while a bigger moment was occurring back home in Los Angeles. The middleweight champion of the world had a choice to make — go to work or go to his wife Alina’s side for the birth of their second child.

The champion went to work, which might be why he’s still champion seven years after winning his first title fight in 18 seconds. Boxing, you see, is an obsessive endeavor. It is a solitary undertaking demanding a singular focus. It is not a sport for someone whose mind tends to wander. To do so is to court not only defeat but the darkness of unconsciousness.

The undisputed middleweight champion of the world is 37-0 with 33 knockouts. He has stopped 23 of the past 24 fighters he’s faced and successfully defended various forms of the 160-pound championship (he holds the WBC, WBA, IBF and IBO versions) 18 times since first winning the WBA portion by knocking out Milton Nunez in the first seconds of their Aug.  14, 2010 mismatch. Golovkin’s approach was the same preparing for that night as it’s been preparing for Saturday night’s far more significant showdown with former junior middleweight champion Saul “Canelo” Alvarez at T-Mobile Arena.

This is a match Golovkin has waited all his life for. At 35, it is his moment. This fight will determine how he will be remembered. It is more than a business meeting. It is, he believes, the moment he will declare that his name belongs among the greatest middleweights in boxing history. Saturday night, all the sacrifice was worth it or it wasn’t.

To give himself the best chance, life must be put on hold. Even birth, the dawning of life, must be ignored, for to lose for even one day that singular focus is to put at risk all he’s worked for since he was taken by the hand by his older brother, Sergei, and delivered to a boxing gym in Kazakhstan at the age of 9. What Golovkin learned that day has become a simple mantra for success in boxing, and in life’s most difficult moments.

“When you are hit, hit back,” he says. It has been his simple philosophy since the morning he first slipped on boxing gloves and made a choice that one day would convince him even to ignore the birth of his daughter.

When his wife called him at his training camp deep in the San Bernardino Mountains around Big Bear Lake to say she was leaving for the hospital, Golovkin was told by his trainer, the aptly named Abel Sanchez, to make the three-hour drive home. It was the final day of full training before he was to throw out the first pitch at Dodger Stadium and then fly the following Monday to Las Vegas. What difference would a day make?

“He said, ‘No coach, we have to train,’ ’’ Sanchez recalled yesterday. “He told me, ‘The baby will come either way. I’ve done my part.’ So he trained that whole day. He wasn’t having the baby. His wife was. This was his business.”

When you work in the “hurt” business, every day counts. When you are facing your biggest challenge, the one you believe will define you for the rest of your life, they count even more. They count because a lifetime of labor is on the line against Alvarez, but Golovkin understands something more is at stake. A life. His own.

“This is not an easy fight for him or for me,” Golovkin said. “Not (a) street fight. Not (a) business fight (a reference to Floyd Mayweather Jr. vs. Conor McGregor). True boxing fight to show who is best, me or Canelo.

“Difficult sport, difficult business. Very dangerous. One punch can change a life.”

Golovkin’s words are no exaggeration. As Mike Tyson once said, “You play football. You play basketball. You don’t play boxing.”

Certainly Alvarez (49-1-1, 34 KOs) is no one to play with. His only loss was by decision to Mayweather four years ago when he was 23, a fight he admits he was too young and inexperienced to take but learned much from. After dominating the 154-pound division, he chose to pursue Golovkin. Although only in his second middleweight bout, even Golovkin concedes Alvarez is the best fighter he’s faced.

If you are wise, you prepare yourself for every challenger, but you might not miss the birth of your first daughter to meet every challenge. That Golovkin did says much about what his mindset has been since the moment the fight was announced four months ago.

That night, Alvarez had just finished beating up Julio Cesar Chavez Jr. for 12 lopsided rounds at T-Mobile Arena when promoter Oscar De La Hoya announced the deal was done. Ever polite, Golovkin said, “Good luck in September.” Alvarez’ reply bore a more hostile tone.

“Luck is for the mediocre, my friend,” he said in an unfriendly way.

Ever since, the two have praised each other as great champions and taxing challenges. Each says their style dictates a night of mortal combat. Pain awaits them both. So does a decision.

At some juncture, as fists are raining in on them, they will have to decide how much victory is worth. Golovkin has trained for that moment all his life. Even on the day his daughter was born, he was asked, “How much is victory worth?”

It was worth even ignoring a moment that cannot be repeated.

“My focus is on boxing right now,” Golovkin explained. “Next week, we can talk about my family. About my life. Next week, I’m a different guy.’’

Until then, Gennady Golovkin is a fighter waiting for his moment, a moment he will face with a focused mind and one thought.

When you are hit, hit back.