Kent 230 and 15 for 4 (Kleinveldt 4-8) trail Northamptonshire 384 (Duckett 208, Wakely 73) by 139 runs

As Northamptonshire ended the day at Beckenham dancing, revelling in how their end of season buoyancy has extended to the County Championship, they had trampled on Kent's promotion hopes. Kent now face requiring something wondrous from the returning Sam Billings to extend their hopes of a return to Division One beyond tomorrow.

In the evening Kent's destroyer was Rory Kleinveldt, swinging the ball late to eviscerate Kent's top order, and locating a perfect yorker to uproot Joe Denly's middle stump. In a half hour of bedlam, he claimed four wickets and only accentuated the brilliance of what had come earlier: 208 magnificent runs from Ben Duckett, born six miles from Beckenham and making Kent rue his family for moving away in his youth.

A little after four o'clock, Kent summoned Hardus Viljoen, their overseas quick and a man who played Test cricket earlier this year, back into the attack. He might have assumed that his pedigree would command respect. Instead, he saw his first delivery scythed through midwicket for four, pulled with contempt off the front foot. The same result met Viljoen's second ball though, to add to the ignominy, this was hit gun-barrel straight, a little to mid-on's right.

In these two balls was the distillation of a singular and rare talent. Duckett is only 21, and yet has spent the summer mocking the notion that the harum-scarum county schedule should act as an impediment to all-format excellence. He now has 2619 runs in all formats this summer, the highest tally in the land.

It is the mark of Duckett's summer that, in the pantheon of his most spectacular innings this year - 220* for England Lions in a 50-over game for England Lions against Sri Lanka A, an unbeaten 163 to lift England Lions to their target of 248 against Pakistan A, a brazen 84 in the semi-finals of the T20 Blast, 282 in the County Championship against Sussex and 180 on a pitch turning viciously against Glamorgan - this double century would not even sneak into the top five.

For vast swathes of this innings, there were little sign of the pyrotechnics that are the hallmark of Duckett in T20, but rather copious impeccably-judged leaves against Kent's seamers, married with scurrying between the wickets and skilful working the ball into gaps. Duckett possesses copious power and 360-degree hitting, yet his greatest quality might just be the dexterity of his wrists, enabling him to manipulate deliveries past fielders placed for the exact shot. While there were a sprinkling of sublime extra cover drives, the most striking thing about Duckett's 149-ball century was the sense of a man playing within himself, of there being more to unleash.

And then it happened: an absurd reverse-pull off James Tredwell, hit with such nonchalance that Kent's fielders could only laugh, followed by a conventional pull the next delivery: same ball, same result, very different means to that end. It was a sign of a man bent on raising hell, and raise hell he did, pummelling Kent with insouciance and inevitability in taking 91 balls over his second century. The only thing that was lacking was a six: perhaps just as well for Mathieu Flamini, who spent the day having a medical at Crystal Palace's training ground, adjacent to the straight boundaries.

This outrageous skill is matched by an awareness of the vicissitudes inherent in batsmanship. Duckett knows that the life of the opening batsman is often nasty, brutish and short - after play, he said that he'd been dismissed for single figures 16 times this year. So he has resolved to strain for every when he is set. Four Championship centuries this season have brought a lowest score of 185. "I'm desperate to score big runs. I could have ended up with four double hundreds this year which would have been unbelievable," he said.

As Duckett's star has soared so, inevitably, have whispers developed that he could leave Wantage Road, just as David Willey did a year ago, despite having a year left of his contract. Yet Northants are confident that Duckett will stay in 2017, which he is already contracted for, and long beyond. They envisage in Duckett not another David Willey, a player who is developed and then effectively sold, football-style, at a profit, but rather a Moeen Ali: a cricketer who rises from an unfashionable county to win England recognition, at which point there is no need for them to leave at all.

"I'm very happy at this club," he said after play. "You can see from all the boys now how happy we are playing with each other. We'll have a beer tonight [he was already nursing one] - we do celebrate very well together and get on well."

As the carnage from Duckett's bat ensued, it would have been easy for Kent to rue their misfortune: not just at coming up against a man in such transcendent form, but also for playing their best County Championship cricket since 2009 in a year in which only one team will be promoted. "In any other season we'd have gone up," chuntered one disgruntled supporter.

Yet, rather than be demoralised by Duckett's assault, Kent displayed fortitude that would make them worthy of a cherished return to the top flight. Chicanery from Viljoen and Darren Stevens with the second new ball accounted for Northants' last six wickets for just 37 runs in 12.1 overs, but this foreshadowed the destruction Kleinveldt would unleash when it was Northants' turn with a new ball.