Maria Sharapova really burst on the tennis scene for the first time after beating Serena Williams as an unknown 17-year-old in the 2004 Wimbledon final. Williams was the defending champion, and for her to lose was one of the greatest tennis upsets of the decade.
Williams has never forgotten that fateful day and has lost just once to Sharapova in their 20 subsequent meetings.
Sharapova is currently on the comeback trail following a 15-month ban for doping and during her absence, she penned an autobiography, titled: 'Unstoppable: My Life So Far'.
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In an explosive extract, released by People Magazine, Sharapova opened up on why she thinks her record with Williams is so poor.
The 30-year-old revealed she believes their bitter rivalry stems back to the aftermath of the 2004 final, when Sharapova saw Serena was visibly upset.
"When the match was over, Serena hugged me. She said something like, 'Good job'. And smiled. But she could not have been smiling on the inside," Sharapova wrote, as per The Metro.
"What I heard when I came into the locker room was Serena Williams bawling. Gluttural sobs. I got out as quickly as I could, but she knew I was there. People often wonder why I have had so much trouble beating Serena; my record against her is 2 and 19. To me, the answer was in this locker room.
"I think Serena hated me for being the skinny kid who beat her, against all the odds, at Wimbledon. But mostly I think she hated me for hearing her cry. Not long after the tournament, I heard Serena told a friend - who then told me - 'I will never lose to that little b**** again.'"
Sharapova believes their rivalry is what has driven Williams to becoming the greatest female tennis player of all time:
"Serena and I should be friends; we have the same passion, but we are not. I think, to some extent, we have driven each other. Maybe that's what it takes.
"Only when you have that intense antagonism can you find the strength to finish her off. Who knows? Someday when all this is in out past, maybe we'll become friends."
Williams, now 35, has won 17 grand slams since she lost in 2004, taking her total to 24. Meanwhile, Sharapova has won just four since her stunning victory over Williams.
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