Her cousins, Princes William and Harry, donned "Team GB" polo shirts and joined an estimated 50,000 less-regal fans in Greenwich Park to root for Phillips as she negotiated her horse over 28 obstacles and a slippery course that claimed a dozen fallen riders.
"The crowd was unbelievable!" Phillips said after she finished Day 2 of her Olympic equestrian debut. "I hope I didn't take any heads off on the way cutting corners."
Hers was one of the top three scores counted in Britain's second-place standings going into Tuesday's final show jumping round with 130.2 points. Germany led with 124.7 points and Sweden was third with 131.4. The United States was fifth with 155.2.
Phillips, a former world and European eventing champion who is 14th in line to the throne, said the only drawback to the raucous cheers was that she couldn't hear the timer ticking on her wrist. Timer or no, her under-the-limit ride added no further penalty points to her 46.1 score from dressage a day earlier and left her tied for 10th place in the individual standings.
Like golf, the low score wins in equestrian eventing.
Ingrid Klimke, part of the gold-winning German eventing team in 2008, was tied for first place in the individual standings with Sweden's Sara Algotsson Ostholt at 39.3 points. Mark Todd of New Zealand, who won individual gold in eventing in 1984 and 1988, was in third place with 39.5 points after incurring a slight time penalty.
Klimke, a skier when not riding, said she was behind midway and knew she had to egg on her horse Butts Abraxxas to keep the pace. "That's why at the first downhill I went like in skiing, felt like 'schussing' downhill. But at the nine-minute marker I knew we would make it," she said.
William and Harry watched from the main equestrian arena, joined by William's wife, Kate, and Camilla, the wife of Prince Charles. Kate wore a white blouse and navy blue jacket, keeping in line with the Union Jack colours the royal family has worn to Olympic events so far.
Seated alongside William and Harry were Phillips' other first cousins, Princess Beatrice and Princess Eugenie, the daughters of Prince Andrew.
Phillips' mother, Princess Anne, herself a former Olympian eventing competitor, watched from the grounds of the twisty, hilly course dotted with 28 fences, hedges and water obstacles in Greenwich Park, London's oldest royal park.
Phillips said her time — well inside the 10-minute three-second time limit — was particularly good given that her horse, High Kingdom, lost a shoe during the ride. That made it tougher for him to keep his footing and make the quick turns needed to complete the 5.7-kilometre course within the time limit.
"He did amazing," she said. "He really grew up and stepped up to the plate."
She mentioned that it was a special day for other reasons too: Last July 30, she married England rugby player Mike Tindall in the "other" royal wedding of 2011 that followed William and Kate's nuptials.
The thrilling cross-country portion of the three-discipline eventing competition is designed to test horse and rider endurance — and guts — and the course up and down the hills of Greenwich Park fit the bill. There were razor-sharp turns in between jumps, blind 2-meter (6-foot) drops and tricky combination fences — each one designed with a very British story behind it.
The first few horses out Monday seemed to handle the course relatively well, but as the afternoon wore on a dozen riders fell from their mounts. The individual leader from dressage, Japan's Oiwa Yoshiaki, fell at jump 20, a steep two-metre cliff with a deceptive second jump right after.
Other horses slipped on the slick grass, costing them time to regain their footing.
Phillip Dutton, from West Grove, Pa., said he decided at the last minute to put longer studs on the shoes of his horse Mystery Whisper, forcing his team to do some quick work.
"Those guys were like a Formula One pit crew," said Dutton, in 12th place.
Several of the riders who fell wore protective vests that inflated much like airbags upon impact. One of them, Hawley Bennett-Awad of Canada, was in the hospital under observation for a concussion and fracture to a bone at the base of the spine. There were no other serious injuries reported among the other fallen riders or horses.
Thousands of people lined the narrow course to watch the horses, getting an unusually close view of them and enjoying a festive picnic atmosphere on the grass. Some riders complained that their horses got spooked by their proximity, others said the closeness of the crowd actually kept their horses straight on the course.
The royals were watching the competition on big screens inside the main arena; the course came into the arena itself for the 16th obstacle, a tricky combination jump that's a miniature version of the Tower of London.
On Sunday, Phillips' grandfather, the Duke of Edinburgh, joined her mother in the VIP stands to watch her earn a respectable score in the dressage portion of her Olympic debut. After cross-country, she'll compete in show jumping Tuesday, the final portion of eventing. The big question is whether her grandmother, Queen Elizabeth II — an accomplished equestrian in her own right — will come to watch.
Margaret Freeman contributed to this report.