King, who won here last year on King Solomon III, now has 45.2 penalties and an advantage of 7.6pts over Bruce Davidson, from the United States, on Heyday as they go into this morning's show jumping.
This means that King William can clobber one fence and still win the contest, but everyone knows that this is his weak phase. It is bound to be a fraught finale because the horse could easily leave more than one rail on the ground.
"I have worked hard to get him used to show jumping, but these days he's very much my fun horse and I've given up worrying about how he's going to jump on the last day. We'll just have to see how it turns out," King said.
Just behind Davidson come two home riders, Paddy Muir on Archie Brown and Tracey Dillon on Joe's Surprise, with New Zealand's Mark Todd a close fifth on Word for Word.
David O'Connor of the United States, who had been just 0.8pt behind King after the dressage section of the competition, made an unexpectedly early departure after Lightfoot fell at the second part of the Middle Lodge at the fifth fence.
O'Connor was quickly on his feet, much to the relief of the American selectors, who have already chosen him to ride Custom Made (with whom he won Badminton in May) in this week's European Open Three-Day Event Championships at Burghley.
However, Davidson and O'Connor's wife, Karen (who have also been chosen for the American team at Burghley) showed top form by finishing clear and within the time to take second and eighth places respectively.
Todd, who finished 13 seconds over the optimum time , could have gone faster on the eight-year-old New Zealand-bred Word for Word. "We were well up on time until I let him cruise through the middle part of the course, so I probably let him ease up too much," Todd said afterwards. "But he's a lovely young horse and I'm really pleased with him."
Word for Word had proved wonderfully accurate for a horse who has only just turned eight. Indeed, Todd had decided to bring him here because Mike Etherington-Smith, the course designer, has hitherto opted for a technical course, demanding spot-on accuracy. This time it required a bolder, more forward-going approach, but Todd's horse proved able to answer these questions equally well.
Ian Stark, who (like King) is expected to be a member of the British team for Burghley, had a splendid ride on The Moose, even without attempting a fast time. He is now lying in 17th place.
"The aim was to give him a good comfortable round," Stark said of his 18-hand mount. "He's very quick-thinking and agile for such a big horse and he seemed to get better and better. He'll make a Badminton horse one day."
Results, Digest, page 23