Maria Sharapova has provided a fascinating insight into her formative years - including an admission that she wore "hand-me-downs" from fellow Russian tennis star Anna Kournikova after moving to the United States at the age of six. 

It was while training at the Nick Bollettieri Tennis Academy in Florida that Sharapova often found herself wearing the old clothes of Kournikova, another prodigiously talented junior player, who was six years older.  

"In those early years, and I'm not exactly sure why, when I needed clothes, I often ended up with Kournikova's hand-me-downs, which - well, skintight animal prints are not usually my thing," she writes in her memoir, Unstoppable: My Life So Far, which went on sale this week. 

Anna Kournikova in action

Clothes aside, Sharapova came through a difficult start to life in America, describing as "intolerable" her two-year separation from her mother. But it was a time which ultimately gave her the fortitude, edge and composure that would help her to win Wimbledon at the age of 17. While Sharapova and her father Yuri were given visas to leave Russia to further her tennis education in Florida's academies, her mother Yelena was denied permission to join them for two years. Just once a week, Sharapova spoke to her mother on the phone, and those calls were short. 

Maria Sharapova: The rising star

In her book, Sharapova discloses how not having her mother around shaped her personality.

"These early years toughened me up. In fact, I think they explain my character, the style of my game, my on-court persona, why I can be hard to beat," writes Sharapova. 

"If you don't have a mother to cry to, you don't cry. You just hang in there, knowing that eventually things will change - that the pain will subside, that the screw will turn. More than anything, that has defined my career. I do not bitch. I do not throw my racket. I do not threaten the line judge. I do not quit. If you want to beat me, you are going to have to work for every point in every game. I will not give you anything. Some people, especially the sort who grew up in country clubs, on manicured lawns, are not used to a girl who keeps on coming." 

Such was the cost of the calls, Sharapova and her mother only spoke for a few minutes each week.

"Was I lonely? Was I sad? I don't know. This was my life and I had no other life to compare it to. I spoke to my mother once a week on the phone. The calls were short, because of the rates. She asked what I was doing and told me that she loved me," writes Sharapova. "I don't really remember the conversations, but I do remember the letters. I wrote to her every day. I'd scribbled at the bottom: 'I love you, I love you, I love you'." 

Eventually, Sharapova's mother made it to America.

"This was maybe the happiest time of my life. You don't realise how much you've been missing someone until you have that person back," Sharapova writes.

When Yelena Sharapova arrived in Florida, and the family started living together in an apartment, she "immediately put things in order", including "throwing out" Kournikova's hand-me-downs. 

Maria Sharapova's book, Unstoppable: My Life So Far, was published this week.