Six riders have secured back-to-back victories since Badminton's 1949 debut. Brit Pippa Funnell was the last to complete the feat in 2003, while countrywoman Lucinda Green has done it twice.
"We could say we won it last year and not to do it again, but that would've been too easy, wouldn't it?" Nicholson said.
"Myself and Nereo are ready to have another go. He doesn't know he's 18. He may have lost a bit of speed, but he still has plenty of stamina.
"It's been a tricky season; wet and cold with a lot of events cancelled. He's done two events and prepared well with a 2nd and a 4th.
"His training gets stepped up day-by-day, similar to most athletes. He's been around a long time and knows when he's got a big thing coming because he get galloped more, and his dressage training gets raised. That's why he's been competitive for so long - the will to show people what he can do."
The pair's success is easy to quantify with 15 starts at four-star, Olympic or World Games events for two wins, seven podiums and 11 top-10 finishes.
It is harder to measure the loyalty built in the 14 years since Nereo was purchased in Spain.
Four-star eventing glory requires hours in front of the mirror in Nicholson's home-made dressage ring, the workouts in the 400ha of woodland adjacent to the family's Wiltshire farm, the gallops at nearby Barbury Castle and 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 after dissatisfaction with the monitoring of Nereo's intravenous drip at the 2014 World Games; three dropped showjumping rails saw the duo's 2015 Badminton hopes descend from overnight leader to sixth.
Nicholson also spent months out of the sport after breaking his neck on August 9, 2015 in the Festival of British Eventing at Gatcombe Park.
Last year Nicholson said he "owed" the Badminton victory to Nereo having "let him down" previously.
As a "creature of habit", the veteran rider has maintained his usual routine ahead of the title defence.
"I realise they put the cross-country course on the computer system a few weeks ago. A lot of riders and the public look at it and analyse it, but I don't.
"I like to arrive [at the venue] on Wednesday morning, walk the course on Wednesday afternoon and use my own eyes and judgement.
"I believe you've got to walk it properly to get the gist of the terrain. You can make similar kinds of jumps in practice, but it's not the same as being there, seeing what state the course is in, and with people all around."
At 56, Nicholson competes in a closer radius to his Marlborough home these days. For example, he decided not to compete at last week's four-star event in Lexington, Kentucky, despite winning on Quimbo in 2013.
"It's a superb event but I'm done with all the rushing. I'm happy to go along and target the events I or my owners want. I find the travelling harder now."
However, Nicholson's hunger remains obvious, despite nine four-star triumphs at Burghley (five times), Kentucky, Pau, Luhmuhlen and Badminton, and three Olympic and three World Games medals.
"I'm very excited about it [Badminton]. I've been going there an awful long time, yet never turned up as defending champion. But that's what we do everything for isn't it? That buzz."
Fellow New Zealanders Sir Mark Todd, Caroline Powell, Tim and Jonelle Price, Dan Jocelyn, Andy Daines and Virginia Thompson will also be competing among the 78 confirmed entries.
