King and Games rookie Nicola Wilson were the only members of the Great Britain team in action with Zara Phillips, Tina Cook and world number one William Fox-Pitt entering proceedings on Sunday.
They lie unofficially third in the team standings while King produced her best ever score with Imperial Cavalier to also sit third in the individual event, while Wilson lies joint 16th with a personal best on Opposition Buzz.
"I'm not going to cry but I'm close to it with the home crowd all around me and so behind me," said King.
"They were particularly quiet and well-behaved - I think the word had spread that Imperial Cavalier is very enthusiastic and energetic and that I needed to keep the lid on him.
"He was a bit tense and we made a few little mistakes, particularly at the last flying change, when I lost him.
"But I am absolutely delighted and I think this was probably the second best Olympic dressage test I have ever done."