NEW YORK -- They share a family and a sport. They have sponsors and coaches in common, too, along with a level of Grand Slam success unmatched by any other active player.
Venus and Serena Williams don't do absolutely everything as a team, though. When it came time Tuesday to prepare for their all-Williams quarterfinal at the U.S. Open, the sisters lined up on adjacent courts, hitting beside -- not with -- each other.
They often practice together. Not this time. Not with what's at stake Wednesday night in their 17th meeting as professionals. Their father, who along with their mother coaches both women, went back and forth, keeping tabs on his racket-wielding daughters.
Later, the side-by-side training sessions done, the sisters caught separate rides away from Flushing Meadows. As Serena walked alone to the parking lot, checking for text messages on her pink cell phone, she was asked if it becomes less tough to have to look across the net during a match and see Venus standing there.
"It does," Serena said Tuesday. "Each time, it gets easier."
Both siblings have defeated the other eight times. The series is also tied at 5-5 in showdowns at major championships.
Their title match at Wimbledon in July -- when Venus earned her fifth title at the All England Club and denied Serena what would have been her third -- was filled with power and precision. It was the quality of play they nearly always manage to produce against other women but have had trouble replicating when facing one another.
Finally, it seems, they can forget temporarily that they're sisters and swing freely.
"It's my career and her career," Serena said Monday night after beating Severine Bremond 6-2, 6-2 in the fourth round. "I know she can definitely bury it, so I can do the same thing."
Venus sounded a similar note after her 6-1, 6-3 victory over No. 9-seeded Agnieszka Radwanska of Poland. Looking ahead to the quarterfinals, Venus said of facing Serena: "That will be a huge milestone -- to get by her to win this championship. That's pretty much how I see it, as trying to win the tournament."
Each Williams won the U.S. Open twice: Serena in 1999 and 2002, Venus in 2000 and 2001. Their meeting in the 2001 title match at Flushing Meadows was the first time sisters played each other in a major final since Maud and Lillian Watson decided the very first Wimbledon championship -- in 1884.
Venus and Serena played again in the 2002 U.S. Open final, part of a stretch in which they met in five of six Grand Slam championship matches. That was back when both spent time at No. 1 and were far and away the dominant figures in women's tennis.
"Obviously we were playing very well then," Venus said. "Things happen. Life happened. You can't always predict it."
Long stretches of inactivity, due in part to injuries, led to slides in the rankings -- Serena is No. 3, Venus No. 8 -- which is why the luck of the draw came into play at the U.S. Open and they were sent to the same section of the tournament bracket.
Instead of continuing to swap major titles the way some sisters swap secrets, they watched Justine Henin, Maria Sharapova, Kim Clijsters and others clutch those trophies and ascend to No. 1. With Henin and Clijsters retired, Sharapova sidelined by a bum shoulder and current No. 1 Ana Ivanovic bounced by a second-round upset, the potential stumbling blocks were fewer at this tournament.
Still, now in their late 20s, the Williams sisters do appear to have returned to prominence.
Serena has lost a total of only 14 games through four matches here, and Venus has dropped 15.
And they're climbing back up the rankings. Serena has a chance to be No. 1 at tournament's end, although she's part of a large group who can say the same. That includes Jelena Jankovic and Elena Dementieva, who will meet in the semifinals after winning Tuesday. Jankovic beat Sybille Bammer 6-1, 6-4, while Olympic singles gold medalist Dementieva defeated Patty Schnyder 6-2, 6-3.
Jankovic and Dementieva, like every woman not named Williams who's still playing, are searching for a first Grand Slam title.
Serena, meanwhile, is after No. 9, and Venus seeks No. 8.
How's that for a sibling rivalry?
Venus was asked before the U.S. Open began whether she and her younger sister can once again be Nos. 1 and 2 in the rankings.
"That's the plan," she said, "but I don't think either one of us is aiming for 2."