Only series leader Mary, or William – currently lying second – can win the $150,000 ( £93,000) prize. With just 13 points between them it will be an intense final.
William – who won the series in 2008 and 2010 – can only beat Mary if he wins and she finishes lower than eighth.
He heads to Pau, in south-west France, as the world number one in the HSBC rankings.
Riders are allowed to compete at Pau with three horses this year, as Mark Kyle did in 2009.
William is taking two nine-year-olds, Oslo and Lionheart (recently first and third in the Blenheim CIC*** for young horses) and the experienced Macchiato.
Mary, who has declared the HSBC FEI Classics to be her major goal this year, rides Apache Sauce and Imperial Cavalier.
Andrew Nicholson is lying sixth, just outside the top five money prizes and could move up with his three experienced rides, Mr Cruise Control, Henry Tankerville and Armada.
Pau is the last four-star event of the season for riders in the northern hemisphere.
But it also heralds the opening of the 2012 HSBC FEI Classics season, which, for the first time, includes Australia’s premier event, the Adelaide International Horse Trials (18-20 November).
We’ll have daily reports from Pau from our Eventing Editor Pippa Roome on www.horseandhound.co.uk from Friday.
And don’t miss the full report in next week’s magazine, out on 20 October.