Maria Sharapova believes her long-running feud with Serena Williams dates back as far as her victory over the American in the 2004 Wimbledon final.
For over a decade, Williams and Sharapova have been two of tennis' most successful and globally recognised stars.
However, while there is a respect for one another's talents on court, there is an apparent enmity off it.
Last month, an excerpt from Sharapova's autobiography "Unstoppable" was published in which the Russian stated: "I think Serena hated me for being the skinny kid who beat her, against all 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'."
And Sharapova, who has a poor 2-19 record against her rival, elaborated in an interview with The Guardian, saying: "I think it goes back to that match.
"Her mindset really changed. There's definitely a level of intensity and competitiveness that comes out much more ferociously against me than other opponents.
"I think I could definitely give my best more often."
Sharapova was also asked why she struggles to respect Williams off the court and appeared to make reference to a 2013 Rolling Stones interview in which the 23-time grand slam champion made a veiled dig over the Russian's then relationship with Grigor Dimitrov - for which she later apologised.
"There are definitely choices in her words, things that she's chosen to talk about that weren't correct in the past," she added.
"One of the interviews. When you start talking about family and other people you don't know anything about, that's when I put the brakes on.
"Those comments weren't necessary."