Williams took victory in the prestigious five fence challenge on Depardieu van't Kiezelhof, picking up four perfect scores to beat Tina Fletcher and Promised Land.
Whitaker won the final competition of the day, taking Arena UK Lando around the course in less than 25 seconds to win the Zinc Management Trophy, finishing ahead of Lauren Hough on Ohlala by less than a second.
Cassie Hartley claimed the first win of the day in the Guy Hawkes III intermediate show hunter of the year on Acheval Boutique Amadeus, finishing just ahead of Louvaine Rooney, ridden by John Cookson.
Chris Orchard secured a comfortable win in the small double harness scurry, finishing over three seconds clear on Carriage House Insurance Touch & Go, while Gareth Roberts took a narrow win in the large double harness scurry, despite amassing four penalties.
In the working hunter pony of the year competitions, there were wins for Sophie Manners with Bronheulog Harvey, Sandra Hammond with Steel Liath, Annabel Prince on Rainbows Mischief and Tinkas Flash shown by Helen Sloan.
Pearl Underwood, Danielle Heath and Bernie White took wins in the show hunter of the year classes, while Terry Wall took first place on Nynwood Fantasia on the only entry in the supreme pony of the year contest.