As one of the greatest tennis players of all time, Serena Williams arguably has no rivals. Just tell that to Maria Sharapova, though, who is convinced otherwise. In her upcoming memoir Unstoppable: My Life So Far, the Russian tennis player reflects on her relationship with her frequent match and reveals some surprising details. One of them is that Sharapova is apparently intimidated by the new mother's strength and physical stature, despite being five inches taller than her.

"First of all her physical presence is much stronger and bigger than you realize watching TV," she writes, according to The Daily Mail. "She has thick arms and thick legs and is so intimidating and strong. It's the whole thing—her presence, her confidence, her personality... Even now, she can make me feel like a little girl."

Never mind that Sharapova famously beat Williams in one of the greatest upsets in tennis history at the 2004 Wimbledon finals when she was just 17 years old (Williams was 23 at the time.) That moment is also reflected on in the book, where Sharapova writes, "I think Serena hated me for being the skinny kid who beat her, against all odds, at Wimbledon. I think she hated me for taking something that she believed belonged to her. I think she hated me for seeing her at her lowest moment. But mostly I think she hated me for hearing her cry. She's never forgiven me for it."

Elsewhere in the memoir, which comes out September 12 via Sarah Crichton Books, Sharapova reflects on her own struggles behind closed doors. "People think it's a glamorous life," she writes. "And, in a way it is. It can also be confusing and lonely...You don't have to put off the healing balm of retail therapy...When you feel you need to see a psychologist, go out and buy a pair of shoes instead. If they're really great shoes, all your worries will evaporate. Why pay $300 for some BS talk with a psychiatrist when you can pay the same and end up with a great pair of shoes that will be with you every day. It's common sense!"