Britain's three-day eventers got their bid for gold off to a great start when two of the five-strong team finished day one of the dressage in the top 10.
King is all smiles after a strong display in the dressage
Mary King and King Solomon III scored 48 to take a share of seventh place, while Jeanette Brakewell riding Over to You recorded 49.8 to move into 10th.
Germany's Bettina Hoy got the top score of 32 for her test on Ringwood Cacatoo.
The United States lead the team standings. Britain are ninth but their three best riders are still to go.
Pippa Funnell, William Fox-Pitt and Leslie Law - are all in action on day two of the dressage.
Law, partnering Shear L'Eau, will be the first Briton in action on Monday, having been scheduled to ride in the opening session.
King, called up as a last-minute replacement for Sarah Cutteridge, whose horse The Wexford Lady went lame, felt she had done better than her result reflected but was still thrilled to be involved.
"I'm generally very pleased, but a bit disappointed about the score. When I finished I thought I had around 38," she said.
"The problem was at flying changes, he couldn't turn from left to right but generally he was very relaxed."
In all phases of the three-day event the winner is the one with the lowest tally.
Regarding her late call-up King added: "I couldn't believe it when my manager (Yogi Breisner) called and told me I'm in the team.
"It is obviously disappointing for Sarah, but I couldn't wait to go out and show the selectors they had made the right decision."
Britain's Angela Tucker is among those judging the dressage at the Markopoulo Equestrian Centre.
Meanwhile, veterinary experts have advised riders to have their horses weighed daily to ensure they are not becoming dehydrated in the hot, dry Athens climate.