In Parker's corner is Kevin Barry, the Olympic silver medallist who took fellow Kiwi David Tua to his world title fight against Lennox Lewis in Las Vegas 16 years ago.
In Ruiz's corner, it is Abel Sanchez, a man who has trained 16 world champions at his legendary Summit Gym, high in the San Bernadino mountains at the Big Bear Lake alpine resort.
Barry and Sanchez, born in Mexico but who has lived in the United States since he was six, have a huge amount of respect for each other, probably because they are similar.
Both are old-school coaches with an emphasis on discipline and training methods which have stood the test of time. Both are also keen students of the sport, with an eye on the latest developments. Both, too, hate to lose.
Sanchez's golf clubs sit in a corner of his gym which is situated in the ground floor of the house he built. He has a handicap of 12 despite not playing often. A qualified builder and in the construction business all his life, he made a lot of money outside boxing and can afford not to coach. He does it because of his love for the sport; the builder taking the raw materials and moulding them into his own vision.
He also has has an artistic streak to go with the more prosaic, having the best photography gear he can get his hands on. He says once he decides to do something, he has to do it to the best of his ability. That's one of the reasons why Ruiz Jr will almost definitely be at his physical best when he steps into the ring against Parker.
Barry is just as driven and there's a burning desire in him to take 24-year-old Parker to the title, something he couldn't quite do with Tua in 2000. He is a father figure to Parker, based in Las Vegas nine months of every year, just as Sanchez is to Ruiz Jr.
The two men have had a bit to do with each other over the years, as is the case in boxing, despite the fact they are based in the separate states of Nevada and California.
"I've seen him a few times over the years," Barry says of Sanchez. "He had one of his guys at the Wladimir Klitschko training camp when we were there last year and we chatted a bit."
Told that Sanchez talked of his respect for Barry and his own dislike of losing, Barry says: "We all strive for perfection. Well, the real coaches do. I'm not good at losing.
"I don't really see it like that [a head to head]. I remember when I was training and managing Tua, and we fought Michael Moorer and he was coached by Freddie Roach. Everyone was staying, 'he's a great trainer, and this is a hard fight', but Dave knocked him out 30 seconds into the first round. It had nothing to do with what was going on between me and Freddie.
"It's not about me and Abel. When the bell goes, it's about two guys with game plans. They will both be well-calculated game plans. It will come down to who is the superior fighter and which one can implement the game plan under the big pressure and the bright lights."
There are few bright lights in Big Bear Lake and Ruiz, in a stable of several fighters including star Kazakh middleweight Gennady Golovkin, has to fend for himself to a certain extent. Living upstairs above the gym, his training times are written on the whiteboard. His life at the moment consists of eating, sleeping and training and, for a man who has battled with his weight during his career, the latter is crucial.
"I've always been a believer that they have to fend for themselves," Sanchez says. "You're a grown man, fend for.yOurself, nobody is going to wipe your brow or massage your shoulders.
"They are going to get into the ring by themselves, they are going to have to understand to deal with this by themselves.
"Do I get nervous before fights? No, never. I don't get hit. I get nervous when I have a lighter guy who is killing himself to make weight. He can't eat, you can't feed him, you see the pain. That's when I get nervous because I don't want to see people get hurt. I have seen three tragedies, not my guys, opponents, and that's because of the struggles of the weight.
"I have had 16 champions. What are their traits? They follow instructions, it's as basic as that. They have to have skills but there are guys with skills who never go past a certain point. David Tua had a lot of skills but he got to a point but everybody around David Tua [thought they] knew more than Kevin Barry.
"Trainers can see something fighters don't. Parker doesn't see what Kevin sees. But Parker has to allow Kevin to guide him, allow Kevin to be right because Kevin is right. He's been there already."
Sanchez's gym, Sanchez's rules. He let light heavyweight Sergey Kovalev go, as the Russian, now a world champion, didn't listen. He is clearly a hard disciplinarian, and so is Barry, but Parker has been welcomed into the Barry house in the Los Vegas suburb of Henderson.
Wife Tanya plays a big part in helping Parker and his stablemate Izu Ugonoh feel at home, and son Taylor is very close to Parker.
"It's really enjoyable for me, maybe not so much on a personal level but on a family level," Barry says. "It gives me great satisfaction having Taylor with me, having him part of this whole journey. When we look at Joe, and he's just 24 years of age, usually when you talk about a journey, you're getting to the end of something. This is just starting.
"The 12 years we had with Tua, we were back in New Zealand occasionally but he was always around my family, we lived in five different houses. The real serious bit when I started coaching him, we had four years in Vegas, the family were all young. So for me, more than anything else, my daughter and sons are all older now. They know it's a big deal. When Tua fought in 2000, I don't think they understood the magnitude of it.
"Even when I took [Beibut] Shumenov to the [WBA cruiserweight] title, it was something that brought a lot of satisfaction but it wasn't anywhere near the same satisfaction as this journey with Joe.
"I try to be a perfectionist. I tried to be the best manager that I could be. I'm not great at delegating, I'm a bit of a control freak - I'll give someone a job and then I'll oversee it.
"Everything has to be perfect. I'm a very good planner. I don't believe things just happen, you plan for success. That particular experience was history-making and it was big and everything but I think the sheer size of that event took away from the real enjoyment for me and, of course, the end result was not a good result.
"I'd like to think I'm a little wiser now. I'd like to think that some of the things I did then, I'm doing differently now. Experience is a great thing. Some experiences break people and other experiences you learn from. I like to think I learned a lot from that experience and that's why a lot of the things I'm doing with Joe now are a result of what I have learned over that period of time.
"I'm not complacent. I'm always trying to look for better training techniques. I'm always trying to think of new boxing combinations, of new ways to do a certain thing. I sleep with multiple notepads in different parts of my room. I'll get up in the middle of the night and write stuff down because I can't get it out of my head."
Sanchez says: "I've been very blessed, but you have to have a passion. It shouldn't be for the money. Fortunately there is very good money now but 95 per cent of the coaches don't make any money. We do it because of something we have inside.
"The lighting up of the face that you see when they win, when they lift a title, or when they do something right in sparring that you've been telling them ... that's the gift that money can't buy."
Patrick McKendry travelled to Las Vegas with assistance from Duco Events.