Whitaker had watched Skelton, on Everest Limited Edition, take seven strides between the first two fences. He went for six strides on Midnight Madness, thereby saving the fraction of a second that he won by.
But he had to shave corners and go like the wind on straighter stretches of the course to keep that advantage over Skelton, who won this classic last year.
It was the fourth King's Cup victory for Whitaker, who last won it in 1992 on Midnight Madness and this sprightly bay gelding will be his mount at the World Equestrian Games, which begin on 27 July in The Hague, and he can never have been on better form.
Though more than five seconds slower than the winner, David Bowen had two clear rounds to finish third on Ben Hur - one place ahead of John Whitaker on Everest Gammon, who had one error in the jump-off.
Mark Todd of New Zealand (who won this year's Badminton Three-Day Event on a last-minute ride, Horton Point) produced another great feat of horsemanship to finish seventh, with one mistake in the first round, on Bond Xtra. It was only his second competition with the horse, which he may now partner in the show-jumping contests at the World Games.
Di Lampard forgot her recent injuries (a broken leg in October and a dislocated finger in May) when she won the earlier Queen Elizabeth II Cup for women riders on Abbervail Dream. It was her biggest win so far and the seven- year-old horse which helped her to achieve it will surely take her to greater victories.
Lampard, 37, had the only clear round of the jump-off to gain an uneqivocal victory. But the formula (also used in the later King's Cup) was confusing, with those on four faults coming in for the second round, which turned out to be the jump-off. It transpired that Liz Edgar, who had been aiming for her sixth Queen's Cup victory on Everest Premiere, had finished second - with her daughter, Marie, in third place on Winstar.
The Irish- bred Abbervail Dream is by far the best horse Lampard has ever ridden and she has continued to nurture him carefully - grateful that his owner, Robert Verburgt, has resolutely turned down all offers for him.
ROYAL INTERNATIONAL HORSE SHOW (Hickstead): The Redland Roof Tiles Championship for The King George V Gold Cup: 1 Everest Midnight Madness (M Whitaker, GB) clear, 41.66secs; 2 Everest Limited Edition (N Skelton, GB) clear, 42.15; 3 Ben Hur (D Bowen, GB) clear, 46.70. Queen Elizabeth II Cup: 1 Abbervail Dream (D Lampard, GB) clear, 63.36; 2 Everest Winstar (M Edgar, GB) 4 faults, 53.09 secs; 3 Everest Premiere (L Edgar, GB) 4 faults, 54.40.Reuse content