A junior world champion with the Japanese team in 2013 and 2014, the lightweight judoka will now come face to face with her former teammates as direct rivals, with the Tokyo 2020 Games on the horizon.
'There aren't many judo players that have switched their nationality from Japan so it was a little strange,' Deguchi tells CNN Sport.
'But my dad is Canadian and I'm half-Canadian, so I thought it would be nice to fight for another country. I wanted to fight for Canada.'
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Born in Nagano, home to the snow monkey and some of Japan's most iconic temples, Deguchi took up judo at the age of just three.
Her grandmother was a hairdresser who counted a prominent local sensei among her regular clients.
'He came in to get his haircut and he scouted me,' Deguchi recalls. 'From that day, I started my judo.'
Her rise up the ranks was swift, with the youngster winning a number of international titles and quickly establishing herself as one of the country's most exciting talents.
With the sport returning to its birthplace at the upcoming Olympics, everything seemed to point toward Deguchi representing Japan.
Canada's most decorated judoka, Nicolas Gill, had other ideas.
'We contacted originally five years ago, before she started competing for Japan,' says Gill, a two-time Olympic medalist who's now the team's high-performance director.
'Her performance at national events in Japan showed signs that she could be a world-class athlete. She chose to represent Japan at that time, but we made it clear that we would always be interested.
'We contacted her a few times over that span of time to validate her interest and finally she accepted to switch.'
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With countries limited to one athlete per weight category at the biggest competitions, Deguchi was well aware Team Canada gave her the best chance of consistently making the squad.
But her final decision was anything but easy.
The International Judo Federation (IJF) dictates that dual-nationality athletes who want to represent a new national federation must undergo a three-year period without participating for their former country in IJF competition.
It meant Deguchi effectively spent a significant portion of her career in the wilderness, watching on as past and future teammates vied for medals around the world.
'We did not see it as a renouncing of Japan, but a confidence in our program and coaches,' says Gill. 'For us, that an athlete of her level would come from Japan, believing she can perform at the Olympic level with our support, gives us a lot pride.'
An unbeaten return
Deguchi, now 22, returned to the fray at last October's Abu Dhabi Grand Slam in the United Arab Emirates, three years to the day since earning gold for Japan alongside the likes of Hifumi Abe and Sarah Asahina at the junior World Team Championships in Fort Lauderdale, Florida.
At February's European Open in Lisbon, Portugal, she won all five of her matches by ippon (the equivalent to a knockout in boxing) en route to gold, defeating Japan's Yuka Watabe along the way.
That, though, was just a warm-up for the following week's prestigious Paris Grand Slam -- an event attended by as many as 15,000 people with an atmosphere unparalleled on the IJF calendar.
There, an unseeded Deguchi came back to haunt her former charges in style, beating top seed and former teammate Tsukasa Yoshida in the final for the biggest title of her budding career.
'I was happy and so excited because this was my first gold medal at a Grand Slam,' she says. 'To get that gold medal at Paris -- a big tournament -- I am even happier.'
'My Canadian teammates were cheering for me. That made me fight even better. If I had to choose one moment in my career I'm most proud of, winning the Paris Grand Slam would be it.'
Gill terms her performances in the French capital 'exceptional,' but Deguchi has since proven far more than a flash in the pan, taking gold at the Pan American Championships in San Jose, California, and remaining unbeaten this year in a Canadian judogi (uniform).
In the midst of a 19-match winning streak, Deguchi most recently took gold at China's Hohhot Grand Prix on May 25, where qualification for the forthcoming Olympics officially opened.
There she beat reigning Olympic champion Rafaela Silva and former compatriot Momo Tamoki en route to the final, only to face a Canadian in the form of teammate Jessica Klimkait.
'Christa and Jessica both absolutely dominated the under-57 kg division,' said coach Sasha Mehmedovic, after Deguchi won all five of her matches by ippon.
'It was great to see two Canadian flags rise in the same category at the medal ceremony. This result demonstrates how strong Canada is in this division at the global level.'
Deguchi continues to live and train in Japan, overseeing her own schedule with the help of her former university coaches.
A 'quiet and very nice person' off the mat -- interested in trivia, animals and human physiology -- she is fiercely determined about her ambitions on it. .
'My final goal is to get a medal at the Olympics,' Deguchi says. 'My hope is to win, that I will get the gold.'
Should the Canadian succeed, it will be the product of a lifetime's dedication and three long years in the cold.
A showdown with her former teammates is almost inevitable, given Japan's domination of the event -- something that would previously have been impossible.
Gill's mantle as Canada's most successful judoka could already be under threat. Not that he's worried.
'My goal as an athlete was always to be Olympic or world champion. I came close a few times, but did not succeed,' he says.
'As a coach, it was also my goal. [London 2012 bronze medalist] Antoine Valois-Fortier came close but also could not yet win gold.
'Now as director of the whole organization, it is still my goal, so hopefully it will come soon!'
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And if it does, it could happen because a former Japanese judoka made history for the Canadians in Tokyo.
'Judo in Japan is part of the culture. Judo in Canada is a foreign sport,' says Gill. 'But Canada loves athletes that succeed at the Olympics and loves great personal stories. So all the ingredients are in place to write a great story in Tokyo!'
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