The pair's success is easy to quantify; 14 starts at four-star, Olympic or World Games events for two wins, seven podiums and 10 top-10 finishes.
Measuring the loyalty built since they debuted at two-star level a decade ago is a more intangible assignment.
The story warrants a film script.
Securing the ultimate in eventing glory justified the hours in front of the mirror in the home-made dressage ring, the workouts in the 400ha (1000 acres) of woodland adjacent to the Nicholson family's Wiltshire farm, the gallops at nearby Barbury Castle and the backyard jumping practice. Equestrianism is not all tweed-set glamour.
The pair have endured hardship. They settled for fourth at the London Olympics after a grand jury decision threw their dressage preparation into disarray with a 10-minute delay for thunder and lightning; Nicholson grabbed a vet by the lapels and shifted him across a corridor due to dissatisfaction with the monitoring of Nereo's intravenous drip at the 2014 World Games, leading to an estranged relationship with Equestrian Sport New Zealand; and the duo's 2015 Badminton showjumping came under scrutiny when, as overnight leaders, potential victory turned to sixth place with three clattering rails.
Nicholson then broke his neck on August 9, 2015 in the Festival of British Eventing at Gatcombe Park. A shattered vertebrae stood between him maintaining his vocation or tetraplegia when his mount, Cillnabradden Evo, failed to clear the last cross-country fence.
"I owed the Badminton victory to him," Nicholson says of Nereo, the horse he began training 13 years ago after buying him in Spain.
"I'd let him down twice there. I didn't give him the best showjumping ride over the first bit of the course [in 2015], and there was the year [2014] I fell off when he made a small mistake and I made a meal of it because I'd lost concentration.
"I've always been fond - and proud - of him. He hasn't found it [eventing] easy. He's a big, strong, powerful horse, whereas event horses tend to be medium-sized, nippy and athletic. But he's reliable and consistent."
In 2015, Nicholson and Nereo jumped last after British rivals William Fox-Pitt and Chilli Morning had gone clear. The echo of an ovation roared out through the entrance as Nicholson and Nereo approached. Nicholson had to pull Nereo away.
The chance to ride third-to-last this year was seen as an advantage.
"The noise [of the arena] was not quite the same. I could've knocked two rails down and stayed in third, so that kept the crowd quieter.
"I wanted to be near enough to the lead [he was 0.8 penalties behind, or less than a time penalty] that if I jumped clear others would have to as well."
The patrons were not mute for long. Nicholson was humbled by a standing ovation as he and Nereo's owner Libby Sellar received the winner's trophy and £100,000 ($188,400) in prizemoney. The reaction from other riders and coaches struck him as "unbelievable".
"I genuinely felt they were pleased for me. Sometimes you win and get a 'well done' but you wonder 'do they really mean that?' This time it felt incredible."
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A curiosity in Nicholson's victory came in his cross-country preparation. Renowned for his reconnaissance, Nicholson was invited to scout the course with the German riders, including world No1 and double Olympic champion Michael Jung.
Before the event Nicholson cited the 34-year-old and his mount La Biosthetique Sam FBW as the combination to beat.
"I enjoy watching riders like him. He's a top horseman and professional sportsman. It's not just a case of him getting on good horses at big events and doing it. He must do the hard yards at home. That shows when he comes to big competitions."
Beating Jung and Sam, who is the same age as Nereo, resonated most for Nicholson.
"They [Nereo and Sam] started at the top together as 10-year-olds at the 2010 World Games in Kentucky. Nereo was third and Sam won. Since then we'd get close, but now we've crept in front."
With his ESNZ and High Performance Sport New Zealand relationships severed, Nicholson will continue to work with the Germans if the opportunity presents.
"I walked the course and told them what I could see, and how I wanted to ride it. When you're dishing out that sort of wisdom to riders like Michael and Ingrid [Klimke, the cross-country overnight leader], it's a good feeling. It makes you feel respected.
"They assured me they were going to watch me on my first horse [Qwanza] on Saturday morning to make sure I wasn't lying," he chuckles.
Nicholson reciprocated by watching Jung's ride on television.
"I had a smile to myself because he took two of the jumps where I took them with Qwanza. When we walked the course, he wasn't originally going that way.
"It might have looked suspect on Saturday night when they were first and second and I was third, but it worked out all right.
"If all goes well, I'll start doing a bit of coaching with them, working around my riding."
Nicholson's decision effectively ends any chance of a reconciliation with the national body. It also fits with a change in mindset since his injury.
Similar accidents to the cervical spine paralyse 98 per cent of sufferers and destroy the lives of others when the delicate but mandatory surgical procedure goes awry.
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Nicholson underwent eight hours of surgery. His wife Wiggy was told her husband had "more chance of winning the lottery with a single ticket" than recovering with such minimal damage.
The Nicholsons had no insurance, having opted out of payments 25 years prior, but the success of their business and the ability to sell broken-in horses meant their livelihood was largely unscathed.
They were also helped by support from Britain's Injured Jockeys Fund, an organisation founded in 1964 which has spent millions of pounds helping more than 1000 former riders.
"I never doubted I would ride again," Nicholson says. "Before I was stuck down on a bed I could move, so in my mind I was all right.
"When I started riding again it felt easy, albeit with a few changes like moving my head more to loosen the shoulder muscles because my head's in a different position.
"Any horse I didn't have complete trust in was given to other riders, as were a couple who jumped in a shape that jarred my neck. I was honest with the owners and they have been happy to stick with me and go with whatever I suggest.
"They're a loyal bunch."
The support of his family and the regeneration of revenue post-accident hardened Nicholson's views against competing for New Zealand.If anything, his work ethic has been galvanised as he develops a range of horses up the four-star chain.
"It's going to take an awful lot of things to change to get me back in that situation [with ESNZ].
"Without any high performance input, I'm managing to do my job well. Why should I turn around and play their game? You can't put a Nicholson on the naughty seat.
"That didn't help.
"I'll carry on doing it [eventing] until I can't be competitive. Nereo's come out of it well. If he tells me in the meantime that he's had enough, he will be retired."
Nicholson's passion remains obvious after adding Badminton to a figurative mantelpiece of four-star triumphs at Burghley (five times), Kentucky, Pau and Luhmuhlen, alongside three Olympic and three World Games medals.
On the Monday after his career's piece de resistance he rode 10 horses he had "neglected" at home during Badminton.
When the Herald spoke to him he was off to get a warrant of fitness for his truck.
"Some might find it odd, but I've got a big competition coming up at Chatsworth this weekend and have four good horses entered.
"It'd be nice to pop off and put my feet up, but life goes on."
