MARLBOROUGH-BASED New Zealander Andrew Nicholson was on course to make history at the Land Rover Burghley Horse Trials today.

Nicholson leads after cross-country aboard Avebury on a score of 46.1 penalties, with the combination seeking an unprecedented hat-trick of Burghley titles.

They won it in 2012 and were then promoted to top spot last year after Nicholson's fellow New Zealand rider Jonathan Paget was disqualified when his horse Clifton Promise tested positive for the banned substance reserpine after topping the Burghley leaderboard.

Nicholson collected just 5.6 time penalties - the nearest among 63 starters to an optimum time of 11 minutes 19 seconds - but he could afford any slip-ups in today's showjumping finale.

Lurking behind him just 3.7 penalties adrift was Australian Sam Griffiths, who is chasing one of British equestrian sport's rarest title doubles.

Griffiths, winner of this year's Mitsubishi Motors Badminton event, is 5.3 penalties ahead of Yorkshireman Oliver Townend, who holds third place aboard Armada.

Townend was the last rider to be crowned Badminton and Burghley champion in the same year, a feat he achieved five years ago, while Paget and Clifton Promise lie fourth, Oxfordshire-based Izzy Taylor is fifth and six-time Burghley champion William Fox-Pitt sixth.

If 42-year-old Griffiths adds a £62,000 Burghley prize to his portfolio, he will also be firmly on course for a £215,000 Rolex Grand Slam of Eventing bumper pay-day.

The Grand Slam is awarded to any rider who lands consecutive four-star titles at Badminton, Burghley and Kentucky, but it has been achieved only once before, by Britain's Pippa Funnell in 2003.

The day, though, belonged to world number two Nicholson, who conjured one of his cross-country masterclasses to set himself up for another tilt at Burghley glory.

"To bring a horse here three years in a row is amazing, let alone to have a chance of winning," he said.

"I started off pretty quietly, but it felt like we jumped a lot of big fences during the first three minutes.

"Avebury has got a great competitive attitude, and he loves the crowds."

Griffiths was held twice on course during his round after other riders' mishaps ahead of him, but he adopted a philosophical stance.

"I was having a cracking round and then got held about five minutes in, which gave my horse a breather," he said.

"It took me a bit to get going again, then I was held for a second time. My horse finished full of running, and to jump clear around Burghley is always a great achievement."

Townend, who was second at Badminton earlier this year with Armada, once again displayed his outstanding horsemanship skills during an impressive season that also saw him gain Great Britain selection for the Alltech FEI World Equestrian Games.

"We all know Armada and what a wonderful cross-country horse he is," Townend said.

"If you put him in the right place and in the right gear, then 95 per cent of the time he will help you."

Paget, though, appears to be out of the running, standing more than nine penalties behind Nicholson.

Back at Burghley after successfully fighting a long battle to prove his innocence, Paget had a provisional suspension lifted in June and he was eventually cleared of any wrongdoing by a Federation Equestre Internationale (FEI) tribunal, although that ruling was only announced last month.

Paget and his legal team proved the positive test was caused by a contaminated feed supplement and that he could not be blamed, although the Burghley disqualification still stood because reserpine - a sedative - was present during Clifton Promise's test.

A total of 40 combinations completed Captain Mark Phillips' course, while there were 13 eliminations and 10 retirements.

Two riders - Britain's Gina Ruck and Australian Natalie Blundell - were taken to hospital after suffering suspected broken legs following falls, while there was one horse fatality.

Orto, ridden by Welsh challenger Sara Squires, hit his right stifle (a joint at the top of the hind leg) while jumping fence 19. There was no fall of horse or rider.

In a statement, Burghley organisers said: "Following transportation to our veterinary hospital facility where a detailed examination, including radiotherapy, was conducted by our expert team of veterinary surgeons employed by Burghley Horse Trials, the decision was made to euthanize him on welfare grounds.

"We would like to extend our sincere sympathy to Sara, the owners Mr and Mrs Budd, and all associated with Orto, such a talented horse."