Phillips' grandfather, the Duke of Edinburgh, as well as her mother, Princess Anne, were in the VIP seats at Greenwich Park as Phillips rode her horse, High Kingdom, through the paces of a standard dressage test to demonstrate the horse's obedience.
"To be here at home is an amazing feeling, and you just want to try and do your best for the team," she said.
She 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, but High Kingdom didn't respond immediately to Phillips' cues.
Despite the bobble, Phillips said she was pleased with the performance.
"It was disappointing about his first change, but his other ones were really good," she said. "But he coped with all the crowd and is only getting better."
Cheers and applause broke out as she rode into the stadium and erupted anew when she finished. Anne, wearing a bright red canvas hat to protect against the strong sun, applauded politely at the end.
Later, the skies opened and heavy rains forced a brief suspension of competition, drawing complaints from some riders who had competed through the rain. Sun and clouds alternated as the competition drew to a close with Germany favoured to maintain its lead in the team category. Australia and Sweden also performed well.
Phillips, a former world and European eventing champion, said High Kingdom's 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.
"He's very chilled," she said of her bay gelding. "That was nothing to do with the crowd. It was just inexperience and getting stronger, and he's getting stronger all the time."
She said she was looking forward to Monday's cross country event, the most difficult and dangerous of the three-discipline competition, which also includes show jumping.
The 28-obstacle course over 5.7 kilometres through the bucolic Greenwich Park is unusually hilly with several tight turns.
"He's quite quick and easy to turn so hopefully it'll be good," Phillips said. "I think he wants to get out there now; he's a bit bored of dressage."
In an indication of the respect the royal granddaughter has in the equestrian world, Phillips was warmly greeted after she competed by Mark Todd and Andrew Nicholson of New Zealand, who were riding later Sunday.
The gold-medal question going into the competition was whether the queen herself would attend Phillips' brief four-minute test. Instead, Prince Philip represented the monarchy. Princes William and Harry, as well as William's wife Kate, are expected to make an appearance at the equestrian event at some point over the next two days.
Fittingly enough, the equestrian event is being held in a very royal venue: Greenwich Park, which dates from 1433, is the oldest royal park in London. The main equestrian arena sits right in front of Queen's House, a 17th-century building designed as a summer palace for Queen Anne of Denmark, the wife of James I.
Horses are a big part of the royal family: the queen is an accomplished and enthusiastic equestrian and at 86 still rides often — and without a helmet.
Phillips' name itself derives from the ancient Greek word "Philippos" — meaning "lover of horses."
Princess Anne is not only president of the British Olympic committee but a member of the International Olympic Committee and a former equestrian eventing Olympian herself.
She competed in the 1976 Montreal Olympics but her horse fell going over a jump during the cross country event. Phillips father, Capt. Mark Phillips, fared better in his Olympic endeavours, winning team gold at Munich in 1972 and silver in Seoul 16 years later. He's now a top coach of the U.S. equestrian team and sat a few seats away from his ex-wife Anne to watch his daughter compete. Phillips' brother Peter also was in the stands to cheer her on.
In a recent BBC interview, Anne acknowledged the pressure British athletes were under competing on home turf.
"I'd hate to be doing it now — that's all I can tell you!" she said.
But her 31-year-old daughter played down the pressure going into the competition, telling reporters earlier in the week she would have no trouble competing with other royals in the stands.
"They're my family. It's not weird," she quipped. But she shied from the question of whether she had received any advice from her grandmother the queen, herself an equestrian enthusiast.
"Do you think I would tell you that?" she said.
Phillips had qualified for the 2008 Beijing Games but had to pull out after her horse Toytown got injured. British equestrian officials have stressed that Phillips is on the team because she's an excellent athlete, not because she's royal.
Phillips is 14th in line for the British throne but she and Peter have very low profiles in the royal family. They hold no royal titles — unique among the queen's eight grandchildren — after their mother turned down the monarch's offer of honours.
Nevertheless both are very much part of the royal family. The queen and Prince Phillip were honoured guests at her wedding last year to international rugby star Mike Tindall, who has been photographed playfully wrestling William and Harry until they begged for mercy.
Tindall escorted Phillips to the stable after her ride, and even took a picture for an Olympic technician who wanted a photograph taken with his wife.
Margaret Freeman contributed.
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