Dazzling shot-making was as scarce as riveting rallies in Sunday's highly anticipated fourth-round match between Venus and Serena Williams at the U.S. Open. And as their errors mounted, the crowd thinned out at Arthur Ashe Stadium, with fans wandering off to lesser courts in search of higher caliber competition.

Those who remained witnessed Venus defeat Serena for the second time this year, prevailing in straight sets, 7-6 (7-5), 6-2, to advance to Tuesday's quarterfinals. But they hardly saw the sisters in peak form.

The Williams sisters have combined for four U.S. Open championships, winning every title from 1999 to 2002. They have combined for two of the three Grand Slam events contested this year, with Serena, 23, victorious at the Australian Open and Venus, 25, winning her third Wimbledon title. Sunday at the U.S. Open they combined for 59 unforced errors.

"I don't think I played my best today at all," said Serena, who drove numerous backhands into the net and grew increasingly winded in the second set. "I don't think Venus did, either. I told her, we were talking in the locker room, just how horrible we played."

The match also lacked the electricity that normally crackles when the sisters take the court. They are the most explosive players in women's tennis, yet oftentimes the result when they're pitted against each other resembles a chemistry experiment that fails to combust.

Part of the problem Sunday, the sisters said in separate interviews, was that they had to play each other so early in the tournament. Their fourth-round match was the earliest they had met in a Grand Slam event since 1998, when they faced off in the second round of the Australian Open.

"It's kind of very, very weird and awkward, bizarre, to say the least," Serena said. "In a later round, it's obviously easier [to get inspired]. I've never played her this early [at the U.S. Open], so it's a little tough."

Their early pairing was the result of their recent dip in the rankings. Serena is eighth; Venus, 10th.

"I think it was distracting for both of us," Venus said. "I'm really dedicated to getting my ranking up. I feel like, wow, I'm a very good player, and there are all these people ranked ahead of me. After awhile, I'm tired of being ranked low. I'm better than number 10."

With Sunday's victory, Venus improved her record against Serena to 7-8. She also notched her first victory over her younger sister in a Grand Slam event since 2001, when she defeated her in the final of the U.S. Open. And she earned a quarterfinal match against the hottest player in the women's game, Kim Clijsters of Belgium, who needed only 42 minutes Sunday evening to oust Maria Vento-Kabchi of Venezuela, 6-1, 6-0.

"She always comes out with her best game against me," Venus said of Clijsters. "I feel like I'll definitely be able to play better on Tuesday. Serena has promised to help me get my game together."

Top-seeded Maria Sharapova also advanced, cruising past Sania Mirza of India, 6-2, 6-1. The hard-hitting Mirza thoroughly enjoyed herself on the U.S. Open's center court but was ultimately undone by Sharapova's precision and her own weak second serve.

Roger Federer, the top seed on the men's side, had a minor hiccup against Olivier Rochus of Belgium in the evening session, but prevailed, 6-3, 7-6 (8-6), 6-2. Federer has yet to drop a set this tournament.

The day's most exciting match unfolded in the shadow of the main stage, on Louis Armstrong Stadium, where two unseeded players -- Davide Sanguinetti of Italy and Paradorn Srichaphan of Thailand -- slugged away for 4 hours 24 minutes before the Italian won, 6-3, 4-6, 6-7 (7-2), 7-6 (8-6), 7-6 (7-5). Neither player spared effort or heart, and the crowd shrieked its approval -- particularly after a 30-stroke rally late in the fifth set sent Srichaphan sprawling face-first onto the blue court.

Sanguinetti finally closed the match with an overhead and trotted to the net to give his Thai opponent a warm embrace and pat on the back. Then he ran to the stands to smother his wife and baby daughter with kisses. "I want to show to my daughter that daddy can really play," said Sanguinetti, 33.

Over at Arthur Ashe Stadium a different sort of family dynamics was at work.

Dressed in complementary shades of lavender and purple, the Williams sisters traded service games to open the first set in their first meeting since March, when Venus snapped Serena's six-match winning streak against her.

Venus was first to break serve, taking the seventh game. She seemed in command after padding her lead to 5-3 but followed with a sloppy service game in which she double-faulted twice, allowing Serena to break back.

"At that point it was all just about sticking in there, which I feel I'm quite good at also," Venus said.

As the momentum rocked back and forth, the crowd seemed conflicted, eager to root on both sisters rather than one over the other. Sporadic chants of "Go Venus!" "Go Serena!" and "Go Williams!" broke out. But they lacked the raw emotion of a New York crowd that's genuinely polarized.

The close first set was resolved by a tiebreaker that ended on a netted backhand by Serena, who banged her racket in disgust. It was the first of a series of frustrated gestures she'd made before the match was over, chiding herself repeatedly for her errors.

Venus broke Serena in the fourth game of the second set. Trailing 2-4, Serena stumbled as she ran down one of Venus's thundering groundstrokes and ended up on her rump. When she got up, she was limping, favoring her surgically repaired left knee.

While she conceded she didn't move well, Serena said she didn't re-injure her knee or ankle during the match.

"I was just having problems at the end because I was moving a lot and stopping a lot," she said. "It always gives me a little trouble after a certain time period."

But the defeat didn't dent her confidence, Serena added.

"Honestly, I don't think there's anyone besides Venus that could have beaten me at this championships," Serena said. "I'm really feeling my shots."

"I don't think I played my best today at all," said Serena Williams, left, who drove many backhands into the net and grew winded in the second set.Top-seeded Maria Sharapova of Russia serves during her 6-2, 6-1 win over Sania Mirza of India at the U.S. Open.