(Yahoo!) With a powerful ace, Serena Williams capped her biggest win of the tournament.

She'll have to win two more in order to complete the calendar Grand Slam and tie Steffi Graf with 22 Grand Slam titles, but the atmosphere inside Arthur Ashe Stadium Tuesday night felt like a U.S. Open final. 

Playing in front of a sellout crowd, Serena outlasted older sister Venus in less than two hours, 6-2, 1-6, 6-3. 

The quarterfinal match drew all of the attention deserved of what was, for at least a decade, sports’ greatest rivalry. The crowd inside showed split loyalty. If anything, Venus was the favorite, possibly because fans seemed to know that this match would not define Serena's legacy in the same way that it could define the second half of Venus' career.

During play, it was silent enough to hear the subway cars braking on the 7-track just outside the grounds. The loudest cheers erupted when Oprah Winfrey and comedian Aziz Ansari appeared on the big screen. When Donald Trump’s face flashed across, the entire stadium finally seemed to be in agreement, booing louder than they’d cheered all night.

Serena earned her first break-point opportunity in the very first game, but failed to convert. She’d finally do so in the fifth game to take the 3-2 advantage, and again in the seventh, to lead 5-2. A quick hold, and she’d taken the first set. It looked like she'd cruise to a quick win. The sisters did not exchange so much as a glance during the changeover. 

When Venus earned a break-point opportunity at 2-1 in the second set, the crowd roared its approval. Venus earned the break, her first of the night. Then she earned another. After playing so solidly in the first, Serena fell apart in the second. It was the opposite of what she's done the other 11 times that she's gone to three sets at this year's majors. Usually she's the one staging the comeback, despite being the consistent No. 1 seed.

"She was at an unbelievable level today," Serena said afterwards. "It's probably the toughest match I have played in a really, really, really long time where I wasn't actually beating myself. I was out there facing an incredibly tough opponent."

In the third, Serena returned to form. A massive ace saved break-point at 2-0. It was soon 5-2. There were few spectacular points or memorable moments in this match, which was watched by more celebrities than Ashe has hosted in the past week combined. Venus owned one set, but Serena owned the night.

"It’s a really great moment. She’s the toughest person I’ve ever played in my life and the best person I know," Serena said in her on-court interview after the win. "It was really difficult today." 

"I think against any other player, she for sure would have won," Serena added in her news conference.

Serena will face Roberta Vinci in the semifinals. Vinci upset Kristina Mladenovic in three sets, 6-3, 5-7, 6-4. She'd earned a place in the quarterfinal after Eugenie Bouchard withdrew from the tournament. In 14 U.S. Open appearances, this is Vinci's first trip to the semifinals. 

Speaking softly in her news conference, Venus discussed the loss' silver lining. "I'm still very excited to see Serena have an opportunity to win the four majors," she said. "That would be a huge [moment], not just for me, but for my family just for what it represents and how hard we have worked and where we come from. 

"But at the same time, if it doesn't happen, it's not going to make or break you. We don't have anything to prove. She has nothing to prove. She's really the best ever."

"I think my success is our success," Serena said. "It is important to me, but at the same time, you know, it is what it is. I'll do what I can."

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