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The 22-year-old from Palmerston had been perhaps just four races away from winning the Women's Motocross World Championships title this season with two races at each of the two remaining rounds - in the Netherlands this weekend and in Italy on September 30 - left to wrap up her 2018 campaign but injury has now dashed her hopes.
Duncan had taken her bike to twice finish runner-up at the fourth round of six in the world series at Ottobiano, in the Lombardia region of Italy, in June, helping her to build a massive 21-point lead over her two main rivals, fellow Yamaha riders Kiara Fontanesi and Nancy van de Ven.
Duncan, who will possibly never enjoy the advantage of racing a WMX event on her home turf, had defied the odds to be in that enviable position, but an injury to a bone in her right foot and then further damage to the ligaments that surround it, have this week forced her to withdraw from this year's title chase.
This will be the third consecutive season Duncan has been cruelly denied.
Duncan finished third overall last year after a contentious jury decision denied her the title when the second race was stopped at the final round in France, while she was leading.
Duncan had also been leading the series during her WMX debut season in 2016, but was forced to the sideline with injury when she crashed into a photographer who had been standing on the race track at the German GP.
Although at that stage the new kid on the block, Duncan stamped her authority and won five of the 14 races in 2016, more than any other individual that year, and this was even after her run-in with the photographer.
Yamaha Racing Team manager Josh Coppins said from Belgium it was "a very bitter pill for her to swallow".
"She had managed her championship campaign so well this year and had almost an entire race in hand over her rivals. This news will be seen as a gift by her competitors.
"This is the nature of our sport, but Courtney is naturally devastated," Coppins said.
"She had injured her right foot at an international event in France in July ... not a GP race ... and the initial diagnosis was that the injury wasn't too severe. She came home to New Zealand to rest and recover between GPs.
"She had another check-up and discovered it was worse than we first thought. She needs to rest her foot now to allow it to heal properly.
"While we may now be looking ahead to 2019, we first have to put this behind us and get over it. We need to make sure she can heal and ensure she is in the best position to be healthy and strong for the rest of her life and so that she can compete again next year."