Fergie previously told PEOPLE that she and Diana would have a “Granny-Off” if the previous Princess of Wales were still alive

Sarah Ferguson and Princess Diana attend the 50th Anniversary of The Battle of Britain Parade on the balcony of Buckingham Palace on September 15, 1990. Photo: Julian Parker/UK Press via Getty

Sarah Ferguson is remembering Princess Diana on what would have been the previous Princess of Wales’ 63rd birthday.

Fergie, as the Duchess of York is popularly known, penned a tribute to Diana on social media on July 1. She shared a photo of the pair standing side-by-side on the balcony of Buckingham Palace to watch the Battle of Britain Anniversary Parade on September 15, 1990, with a heartfelt message: “Happy birthday to my dear friend, Diana. You were a pillar of light and love. And what a legacy you have left behind. I will forever remember our laughter and the kindred, kind spirit I found in you. I am sure you are watching over us always.”

“Rest in peace, my friend,” Fergie, 64, concluded.

Princess Diana and Fergie became royal sisters-in-law through their respective marriages to the future King Charles and Prince Andrew, but they were close friends since their teenage years.

“We were best friends from when she was 14 and I was 15,” Fergie, who affectionately called Diana “Duch,” exclusively told PEOPLE in 2021.

Sarah Ferguson; Princess Diana.Jo Hale/WireImage; Anwar Hussein/WireImage

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In fact, it was Diana who introduced her friend to Prince Andrew.

“I said to her, ‘Oh, Andrew’s really good-looking,’ ” the Duchess of York recalled. “And she said, ‘Duh, Fergs!'” (Sarah and Andrew wed in 1986 and divorced a decade later but remained amicable friends and co-parents of their daughters, Princess Beatrice and Princess Eugenie.)

As part of the royal family, Diana and Sarah would compare notes and offer each other support.

“Diana and I both had our own mental health issues, and she and I used to talk,” the Duchess of York recalled. “She said, ‘Fergie, remember one thing: When you’re at the top of the pedestal, it’s so easy to fall off. And you’re at the bottom. You just climb up.’ “

“We were positioned as saint and sinner,” she continued. “And the most important thing was to remain robust together, and we did, no matter what anyone wrote.”

Sarah Ferguson and Princess Diana at the Guard’s Polo Club in June 1983.Jayne Fincher/Princess Diana Archive/Getty Images

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Fergie said during a 2023 appearance on Good Morning America that Princess Diana would be “very proud” of her five grandchildren from her sons, Prince William and Prince Harry.

“I think we’d have a Granny-Off,” said the Duchess of York, a grandmother to Princess Beatrice and Princess Eugenie’s children. “Because she would run faster in the races. I wonder — she’d probably be funnier. No, I think I’d be funnier.”

Prince William and Prince Harry unveil statue of mother Princess Diana on July 1, 2021. DOMINIC LIPINSKI/POOL/AFP

In honor of what would have been Princess Diana’s 60th birthday on July 1, 2021, her sons, Prince William and Prince Harry, reunited at their childhood home of Kensington Palace to unveil a new statue of their mother. The tribute, located in the Sunken Garden, showed Diana with three children, reflecting her work to support and nurture children around the world.

“Today, on what would have been our Mother’s 60th birthday, we remember her love, strength and character – qualities that made her a force for good around the world, changing countless lives for the better,” the brothers said in a joint statement. “Every day, we wish she were still with us, and our hope is that this statue will be seen forever as a symbol of her life and her legacy.”