Princess Eugenie has told Sky News the Queen would have found the final two days of the Diamond Jubilee "quite testing" without Prince Philip by her side.
Speaking exclusively to Sky ahead of her 100km fundraising cycle in the early hours of Sunday, she described her grandparents as a "power couple who kind of astound people".
She said: "To see Granny and Grandpa standing for four hours, I mean, together, they are invincible.
"They are the most incredibly supportive couple to each other. Grandpa was unfortunately taken ill and for Granny to come and do that alone was probably quite testing and I think he is her rock, really, and she is his."
In the first interview with a member of the royal family since the Diamond Jubilee celebrations , Princess Eugenie looked back on four days she described as an "unbelievable experience that will forever stay with me".
"To see the showing that people gave (The Queen) is just unfathomable for me. I just think it's unbelievable and it makes me so proud," she said.
Since the Jubilee celebrations, the princess has turned her attention on her new Patronage, an appeal to raise money for the Royal National Orthopaedic Hospital.
She said: "At 1am on Sunday morning I'm going to be doing a 100km bike ride around London for my RNOH appeal and the MCC Foundation Appeal.
"When I was 12, I had an operation at the Royal National Orthopaedic Hospital and they corrected the curving in my spine.
"I've got two 12-inch rods and eight screws going up my back keeping me straight and they fuse together with my spine so now I can't live without them."
The princess told Sky suffering from scoliosis has made it easier to relate to patients she meets.
"One girl lit up when she saw me. I showed her my scar and pulled my top down. She said, 'Oh my God, look at you, you're standing,'" she said.
"She was lying down at that point, I think. For me to be living proof for them gives them much more confidence and hope that it's okay and we can get through this."
The 22-year-old university student admitted there have also been lighter moments since her back operation.
"It is sometimes quite fun because my friends can put magnets on my back and they kind of hang off. It's really weird," she said.
Princess Eugenie travelled the length of the pageant on board the historic Havengore barge alongside her sister Princess Beatrice and father, The Duke of York.
Her mother, who was not part of the celebrations, tweeted: "Sooo proud of my girls smiling broadly, celebrating Granny with the nation and Eugie with her flag and both happy smiles."
The princess told Sky: "Mum is so supportive of me and Bea and we do everything as a family.
"For us to be able to come out here and smile and to support Granny with the support of my mother who keeps us going and gives us confidence with everything we do."
Rubbing her hands with excitement, the princess also revealed one of her highlights was the pop concert in front of Buckingham Palace .
"It was phenomenal," she said. "The lighting, Madness on the roof, and the best was Robbie Williams doing that shout out for me and my sister.
"The whole thing was just an unbelievable experience that will forever stay with me because I got to actually be there and take part and see and hear everything.
"The amount of people who came out to support was amazing and that exceeded my expectations more than anything because it just showed how much everyone loves my granny."