Zara Phillips scored a solid 48.10 penalty points, despite a mistake toward the beginning of her canter in the first "flying change".
In the stride, the horse is supposed to change the sequence of his steps from side to side, but High Kingdom didn't respond immediately to Phillips' cues.
Cheers and applause broke out as she rode into the stadium and erupted anew when she finished.
Phillips said it was an "amazing feeling" to be part of the Olympics and to ride for your country and to just be here. The crowd are amazing.
It was incredible to be selected for the Olympics is an amazing feeling. You try to do your best for the team".
Princesse Anne, was wearing a bright red canvas hat to protect against the strong sun, applauded politely at the end.
Phillips, a former world and European eventing champion, said the mistake had nothing to do with the enthusiasm from the stands - a rousing welcome that prompted the announcer to remind the audience to keep applause to a minimum until the test was finished.
The 13th in line to the throne has never competed in an Olympics because of injuries to her former horse Toytown in 2004 and 2008.
But she is now one of the favourites for a medal as she tries to emulate her father, Captain Mark Phillips, who won Olympic gold in 1972 and silver in 1988.
Zara Phillips, who was named BBC Sports Personality of the Year after being crowned eventing world champion in 2006, will compete in the individual and team three-day eventing competitions.
- Tags: Zara Phillips Members Royal family Queen's granddaughter Olympic equestrian debut Phillips' grandfather Duke of Edinburgh Princess Anne VIP seats Greenwich Park eventing dressage competition 48.10 penalty points mistake flying_change stride horse sequence steps side High Kingdom Phillips'_cues Cheers applause stadium amazing feeling Olympics country crowd are amazing team wearing bright red canvas hat sun politely former world European champion enthusiasm stands 13th throne injuries Toytown 2004 2008 favourites medal father Captain Mark Phillips Olympic gold 1972 silver 1988 BBC Sports Personality Year 2006 individual three-day eventing competitions.
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