Wimbledon Lawn Tennis Championships - Day Eight
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Venus and Serena Williams's on-court sibling rivalry is arguably the greatest in sports history. Since they started competing against each other in the majors 17 years ago, they have faced each other 26 times, 13 in Grand Slam events and eight in Grand Slam finals, with Serena winning seven out of their last eight meetings. Now, they're set to play each other tonight in the quarterfinals at the U.S. Open and there's more on the line than ever.

So far, Venus, who is ranked 23rd in the tournament, has advanced smoothly in the Open—she finished off her fourth-round opponent Anett Kontaveit in just 50 minutes. With four solid wins under her belt so far, she's on track to end her seven year Grand Slam dry spell. Serena also has a lot at stake—if she wins the U.S. Open this year, she'll sweep the 2015 majors, earning the first Grand Slam since Steffi Graf in 1988.

Both want to win, but eliminating your best friend and sister from a major is never easy, especially as Serena pursues history. “I don’t think anyone wants to be a spoiler," Venus told reporters Sunday after her fourth-round win. “I think people love to see history being made. But at the same time, you’re focused on winning your match."

Focus they will. Both Serena and Venus will have to push aside their camaraderie to move to the next round, which is made even more difficult as the two are so attuned to each others strengths and weaknesses. "I'm playing the best player in the tournament," Serena told reporters after winning her fourth round match against Madison Keys. "She’s beaten me so many times. She’s a player that knows how to win, knows how to beat me, and knows my weaknesses better than anyone."

While we'll have to wait until tonight to see who will come out on top, one thing's for sure—it's bound to be a bittersweet victory. Watch the Williams sisters in the quarterfinals tonight at 7 p.m. ET on ESPN.