-- Between them, sisters Venus and Serena Williams have fended off a career's worth of injury and ailments this season, including sprained ankle, strained shoulder, sore left knee, stomach disorder, flu and general fatigue. They've also juggled more careers than most people do in a lifetime, including interior decorator, fashion designer, author, TV actress, corporate ambassador and professional athlete.

But for all the skepticism over their fitness, and for all the criticism over their dalliances off the tennis court, the Williams sisters have managed to win two of the three Grand Slam events contested this year. Serena claimed the Australian Open; five months later, Venus hoisted the trophy at Wimbledon.

If their 23 combined seasons on the pro tour have established one truth, it's that the sisters play best when the stakes are highest. And on Friday, both took another step toward the season's final major, advancing to the U.S. Open's fourth round with straight-set victories over opponents who weren't remotely up to the task.

Serena was first to book her spot, overwhelming Francesca Schiavone of Italy, 6-3, 6-4, in an afternoon session marked by blue skies and balmy weather. Venus dazzled under the lights at night, dismissing Slovakia's Daniela Hantuchova, the 20th seed, 6-3, 6-3, before a record crowd of 23,353.

But only one Williams sister will be around next week to chase the family's fifth U.S. Open title. Assigned to the same quarter of the draw, they'll meet Sunday for a place in the quarterfinals. It's the earliest the sisters will have met in a Grand Slam event since 1998, when they faced off in the Australian Open's second round. And it's a result of their recent slide down the rankings. Both former No. 1s, Serena is currently ranked eighth; Venus, 10th.

While Serena called it "extremely disappointing," Venus bubbled with enthusiasm once her victory ensured Sunday's familial showdown, teasing a sellout crowd that included Taye Diggs, Nicole Kidman and Tony Bennett during her on-court interview: "Are you guys ready for the Williams sisters? We'll see you on Sunday! It's going to be a lot of hard serving and a lot of hard hitting. We'll both have some tricks up our sleeve."

Also advancing Friday: Top-seeded Maria Sharapova, who has yet to lose a set; fourth-seeded Kim Clijsters; American Taylor Dent, who earned a fourth-round meeting with third-seed Lleyton Hewitt; and top-seeded Roger Federer, who fended off a spirited challenge from Fabrice Santoro of France.

The Williams sisters' matches have always been a bit awkward and understandably so, given that they are best friends, share the same homes, practice together and spend most of their time together.

Serena holds an 8-6 edge and had won six in a row until late March, when a resurgent Venus defeated her in Miami. They were on course for a fourth-round meeting at Wimbledon in June, but 85th-ranked Jill Craybas derailed the rematch by bouncing an unfit Serena in the third round.

Venus won the tournament in emphatic fashion, toppling the world No. 1 and 2 players, Lindsay Davenport and Maria Sharapova, en route to her third Wimbledon title.

But neither sister played much in the six weeks that followed, taking a hiatus to reknit and recover. That has only added to the fascination with their apparent return to form at the U.S. Open, where neither has lost a set.

"She's playing unbelievable," Serena said of Venus. "I'm just going to have to pick it up."

Serena wasn't as fleet afoot Friday as the scrappy Schiavone, who won several points on drop shots. Serena also committed more unforced errors (22) than winners (16). But she raised her intensity after a lapse in the second set to close the match in 72 minutes. "She's big, but she is not so slow," Schiavone said, asked to gauge Serena's fitness.

Serena's serve was also effective; she landed 68 percent of her first serves and fired four aces. Combined with her two aces on Wednesday, that raises Serena's charitable donation on behalf of hurricane victims to $600 following her announcement that she would contribute $100 for each ace she hits for the rest of the season.

The gesture has received mixed reviews. Serena was the first tennis player to commit to making a donation; the same day, Venus confessed she hadn't heard about Hurricane Katrina and didn't watch the news. "Sometimes," she said, "it's just better not to know."

On Friday Serena acknowledged she's reconsidering her formula, given that she's "not hitting that many aces." She also revealed that the $40,000 diamond earrings she has been wearing during her matches would be auctioned off after the tournament to benefit flood victims. While Serena doesn't own the earrings, she said the designer had intended to give them to her. Finally, she took issue with criticism that characterized her, along with other athletes, as spoiled and self-absorbed.

"I've never been spoiled," Serena said. "To this day, I refuse to buy a car. I want a Range Rover very bad, but I don't feel as I deserve it. I refuse to spend the money to buy a Range. I don't live the elaborate life. The diamonds are borrowed. I won't buy them because I'm too cheap."

On Friday Venus joined U.S. Fed Cup captain Zina Garrison in launching a program to raise money through the sale of yellow shoelaces that symbolize a prayer for hope. "Everyone's in my prayers," Venus said. "I feel like God will watch after them in whichever way."

Venus Williams defeats Daniela Hantuchova to set up her 15th career meeting with sister Serena Williams, who routed Francesca Schiavone.