Now comes the match that Serena Williams didn't want to play.

As she plows forward toward a fourth consecutive U.S. Open title and a calendar year Grand Slam, Williams runs into the player whom she considers to be the best in the tournament in the quarterfinals tonight, the one player who might have the best shot at beating the No. 1 player in the world.

That would be her sister, Venus.

"For me, I'm playing the best player in the tournament, and that's never easy," Serena said. "She's beaten me so many times. I've taken a lot of losses off her, more than anybody. She knows how to win, knows how to beat me, and knows my weaknesses better than anyone."

This will be the 27th time the sisters have played each other. That figure is extraordinary not only because they are the most successful sister-sister act in the history of sport, but they are also the oldest players in the women's draw. Venus is 35, Serena 33 and will be 34 this month. Their rivalry in championship tennis goes back to the 1998 Australian Open, a match won by Venus. Their all-time head-to-head record is 15-11 in favor of Serena, and Serena holds a 6-2 advantage in Grand Slam finals.

Serena Williams has won 21 Grand Slam titles and if she wins the Open, she will tie Steffi Graf's total of 22 and be two behind the record of 24 set by Margaret Court. Venus has seven Grand Slams titles, and beat her sister in the 2001 Open final, the first Saturday night final at the Open.

But all these numbers mean little to both of them as they prepare to face each other. Now, it's only about holding serve or breaking it, hitting winners or making errors, toughing it out to the bitter end, something that both sisters are supremely accomplished at.

Asked to match up each other, Serena responded concisely:

"She's fast, I'm fast. She hits hard, I hit hard. She serves big, I serve big."

In the galaxy of women's tennis, of big, strong, physically tuned athletes, Venus and Serena Williams, at the combined age of 68, remain the most powerful players in the game. When you consider that Venus battles not only much younger opponents, but also her own body, her accomplishments stand out beyond the winning of matches and titles.

At the Open four years ago Venus withdrew and announced she had Sjogren's syndrome, an autoimmune disorder that causes fatigue, dryness and glandular problems. She has never used it as an excuse for failing to win and is reticent to even talk about it.

"I believe everybody has their challenges," Venus said. "I have mine. I put my best foot forward, whatever foot that is. So left, right, whatever. I'm definitely not here to talk about what's wrong or what's not wrong. Everybody's got problems. You get out here and play. If it doesn't work out, you go and work harder or go home. That's it."

Venus' road to the quarterfinal has been considerably more difficult than Serena's, starting with a first-round match against Monica Puig that went three sets and took 2:40. She also defeated rising Swiss star Belinda Bencic, a convincing two-set victory that seemed to prove she was at the top of her game as well as being at the top of her age bracket.

Serena had to beat Venus in the round of 16 at Wimbledon in July on the way to winning the championship. That said, she generally would defer to her big sister.

"I would rather lose to Venus as opposed to anyone else." Serena said.

But?

"I, in general, don't like to lose."

As young girls growing up in Compton, California under the tutelage of their father, Richard, Venus was older and bigger and completely dominant. During their professional careers, Serena has risen to the heavenly heights, and it's the rivalry with her sister that's helped get her there.

"I think she helped create me, for sure," Serena said.

Still, from the baseline of life, they remain sister-sister.

"I feel like Venus and I have definitely proven that you can be friends and you can be sisters," Serena said. "You can be enemies on the court, and you can be friends and sisters off the court."