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has faced off against her big sister Venus Williams dozens of times professionally since their childhood days playing tennis in Compton, California. But despite her impressive track record, Serena says Venus is her greatest opponent.

“She’s the toughest player I ever played in my life, and the best person I know,” Serena told ESPN in an on-court interview after she defeated her sister at the U.S. Open quarterfinals Tuesday night. “So it’s going against your best friend and going against, for me, the greatest competitor in women’s tennis.”

Serena triumphed over her big sister in a 6-2, 1-6, 6-3 win in the tournament at Arthur Ashe stadium in New York. Venus, 35, fought hard in the emotional three-set match and Serena appeared to hold back tears as she served the 107 mph ace that ended the match (and landed her in Thursday’s semifinals against Roberta Vinci).

The two then met at the net and embraced. With her arms around her younger sister, Venus whispered, “so happy for you,” the Associated Press reports.

Serena holds a 16-11 lead over Venus in overall professional competition, and although they’ve met in competition 27 times, Serena says she doesn’t look forward to playing her big sis.

“The only player in the draw I don’t want to play, not only because she’s my sister, but for me she’s the best player,” Serena said, according to CNN. “She has beaten me so many times.”

Serena’s 16 victories over Venus include nine Grand Slam encounters and three out of five U.S. Open matches, CNN reports. The win on Tuesday is a big deal for the no. 1 ranked tennis star, as it brings her just two matches away from a calendar year Grand Slam – a feat that hasn’t been accomplished since 1988.

But Serena tells ESPN that her win in the quarterfinals was not just for her.

“This was a big moment for Venus and I,” Serena said. “It’s a Grand Slam, so we both want to do the best we can.”

The 35-year-old Venus made similar statements to the AP, speaking highly of Serena.

“That would be huge, not just for me, but for my family, just for what it represents and how hard we have worked and where we come from,” Venus said of the historic calendar Grand Slam her sister may achieve. “But at the same time, if it doesn’t happen, it’s not going to make or break you.”

“She has nothing to prove. She’s the best ever.”