Combined, they've played in 18 Grand Slam finals. They've spent more than four years ranked No. 1. They've won both the U.S. Open and Wimbledon four times and have amassed more than $25 million in total prize money.

But for Venus and Serena Williams, the next match at this U.S. Open, at just the beginning of the second week, may be among the biggest of their careers.

Third-seeded Serena beat Patty Schnyder on Sunday, 6-4, 6-2, setting up a monster two days for the Williams sisters. Monday, No. 11 Venus plays No. 5 Lindsay Davenport in the fourth round, and Serena will face No. 8 Jennifer Capriati in a quarterfinal Tuesday.

The sisters haven't won a Grand Slam in more than a year. Once the two most-feared players in women's tennis, they've been humbled by injuries and inconsistent play. Yet they still claim to intimidate; they still swear they can dominate.

"This is the U.S. Open, so this is a chance to prove it," Serena Williams said. "We're still capable of doing the same things we've always done. I mean, we haven't been playing well because of injuries. That doesn't mean we can't play like we used to."

Serena looked every bit as dominant as she did in previous years Sunday. She made 15th-seeded Schnyder, typically one of the game's most consistent players, look incapable of returning serve. When Williams made her first serve, she won the point 81 percent of the time.

During one stretch in the second set, Williams won 10 service points in a row.

She hit her first serve 115 mph, forcing Schnyder to return from several feet behind the baseline.

"When she serves that well, I'll never be able to make a break," Schnyder said. "If she serves like this, it's hard for anybody. If I want to beat her, she has to make a lot of mistakes. Everything has to go my way."

"If I'm serving better, my whole game gets better," Serena Williams said. "And my serve was really on."

Neither sister has lost a set during this U.S. Open, but they've looked far from infallible. Serena has struggled with her groundstrokes, particularly her backhand. In her third match, against Tatiana Golovin, she had more than 40 unforced errors. "I can't believe I had that many," Serena said. "That's just totally pathetic. That's my major problem."

Venus Williams has one, too: her serve. On Saturday night, Venus seemed to lack all confidence in her high, swooping serve, once the linchpin of her power game.

In the first set of her win over Chanda Rubin on Saturday, Venus served for the set three times -- and lost all of them. Eventually, she won the set in a 7-4 tiebreaker.

Afraid of double-faulting, Williams lightly pushed in her second serves. Rubin sent them back a bit more forcefully, turning a dozen second serves into return winners.

"That's the one thing that's still not where I want," Venus Williams said. "But other than the serve, I'm happy. I'm returning a lot better. My game's better. I'm moving better. I'm not rushing as much. I'm starting to play well."

She hasn't felt this optimistic since early in 2003, when she advanced to the final of the Australian Open, marking her fourth consecutive Grand Slam final.

A month later, though, she strained a left abdominal muscle, a nagging injury that would cause her to miss five tournaments and drop from No. 2 in the world to No. 11.

Health problems have plagued her again this year. She sprained her ankle badly two weeks before the French Open, limiting her mobility for that tournament.

She hurt her wrist a month later, an injury that forced her to play with pain at Wimbledon and in the Olympics.

"I was not prepared, and I wasn't fit," Venus Williams said. "It's a terrible thing.

"The injuries hurt my game a lot. I mean, I'm a former number one, a champion, Venus Williams. People come out there and they start to think they can win because I'm not on top. Nobody knows what that feels like."

Except maybe her sister.

Serena Williams also has struggled through injuries. Then the No. 1 player in the world, Serena underwent surgery to repair a partial tear of a tendon in her left knee in August 2003. The injury continued to bother her as recently as last month, when she pulled out of the Olympics at the last minute.

"I'm really proud of how I'm holding up," Serena Williams said. "Doctors told me not to come here. I wasn't supposed to come, but I couldn't miss it for the world. In my mind, I'm recovered."

She'll get a chance to prove it against Capriati, who fought off a set point in the first set Sunday before beating Ai Sugiyama, 7-5, 6-2. Capriati played well through four rounds, and she beat Serena Williams in the quarterfinals of the French Open this summer.

Venus Williams has an equally tough opponent in Davenport, who hasn't lost since the semifinals of Wimbledon. Since then, she's won four consecutive tournaments, establishing herself as the hottest player on tour.

"When we're playing well, normally we're going to be in the winner's circle," Venus Williams said. "That's always our goal, and it hasn't changed."

U.S. Open Notes: Andy Roddick didn't break his record 153-mph serve, but his serve looked better than it has all tournament in a straight-set win over Guillermo Canas. Roddick served 21 aces without double-faulting.

"I had a game plan, and I played really well," Roddick said. . . .

No. 10 Vera Zvonareva lost in three sets to No. 6 Elena Dementieva, and she didn't take defeat gracefully. Zvonareva cried through most of the third set, which she lost, 6-3. "I'm just a very emotional player," Zvonareva said.

"Other than the serve, I'm happy," Venus Williams said. "I'm returning a lot better. . . . I'm moving better."Serena Williams beats Patty Schnyder, 6-4, 6-2, will face Jennifer Capriati in the quarterfinals. Venus Williams will face Lindsay Davenport in 4th round.