MELBOURNE, Australia — For one set, Venus Williams looked every bit the reigning Wimbledon champion.

Then, the rust of a long layoff caught up with her in a flurry of errors Monday as she was knocked out of the Australian Open in the first round by Bulgaria's Tszvetana Pironkova 2-6, 6-0, 9-7 in 21/2 hours.

"I couldn't get it right today. But in general, I am playing really well," said Williams, who hadn't played at tour level since late September. "It's just like, `Wow, it was the wrong time to hit wrong.'"

Williams wasn't the only one to struggle in the season-opening Grand Slam event.

Her sister, defending champion Serena, had her own lapses and survived a major workout from 52nd-ranked Li Na of China, who frequently had her breathless from running sideline to sideline.

Serena, seeded 13th, was broken as she served for the match at 5-4 in the second set. She double-faulted twice at 15-30 and won only one point in the tiebreak, but settled down and finished off the 6-3, 6-7 (1), 6-2 victory.

"Everyone chokes. I choked today," Serena said of the second set. "I didn't do what I needed to do. Li played some great shots."

Top-seeded Lindsay Davenport needed a few games to find her groove before advancing with a 6-2, 6-1 win over Australian wild-card entry Casey Dellacqua. Masters Cup champion David Nalbandian was cruising, then had to fend off a gutsy comeback attempt by Thai qualifier Danai Udomchoke to win 6-2, 6-2, 1-6, 6-7 (4), 6-1.

Meanwhile, fourth-seeded Maria Sharapova had an easy time and showed no signs of her sore right shoulder bothering her, smacking 20 winners while downing Germany's Sandra Kloesel 6-2, 6-1. Sharapova said she has seen dozens of doctors during the last six months and is convinced the problem won't get worse from playing.

Venus Williams, the No. 10 seed, wasn't sure if she would stick around to cheer sister and defending champion, Serena. She committed a stunning 65 unforced errors — 41 in the tense final set in which she was broken while serving for the match at 6-5 — to 22 for Pironkova. She seemed to lose her rhythm after bashing an overhead out that would have given her double break point in the first game of the second set.

"I just seemed to get to a point there, I just couldn't pull my game together," Williams said. "I don't know what happened. I just was struggling to keep the ball in today. Obviously, she benefited from my largesse."

It was only the third time in 34 Grand Slam tournaments that she has lost in the first round. The last time was the 2001 French Open.

"If I had just 10 less errors, I think this match is a different story," Williams said. "Obviously, she's a very good player and, you know, she stuck in there."

The 18-year-old Pironkova, ranked 94th, said Williams had been one of her idols, but she managed to put that aside. After a nervous start, she relaxed in front of a packed Vodaphone Arena that included a batch of rowdy Bulgarians who waved the national flag and cheered her loudly.

"I always loved her game," Pironkova said. "When I go on court, I should not think about that. I just have to play tennis, and I did. I can say truly I did my best today."

Davenport, seeking her fourth Grand Slam singles title and first since the 2000 Australian, set up three match points with an ace and clinched it in 57 minutes with an overhead winner.

Davenport next faces Croatia's Karolina Sprem, a 6-4, 6-2 winner over Alona Bondarenko of Ukraine.

Davenport spent all but seven weeks atop the rankings in 2005 and reached the finals here and at Wimbledon.

She has a new coach — David DiLucia — and a difficult draw at Melbourne Park, with 2004 U.S. Open champion Svetlana Kuznetsova and French Open champion Justin Henin-Hardenne in her bracket.

"I feel like I'm ready for the challenges," Davenport said. "I feel like I have the ability to play well and hopefully repeat what happened last year, and go better."

Eighth-seeded Justine Henin-Hardenne of Belgium, the 2004 winner, beat Marta Domchowska of Poland 6-2, 6-2.

Former Wimbledon semifinalist Jelena Dokic, a wild card in her first Australian Open since 2001, crumbled after thinking she'd won in straight sets. Dokic celebrated a forehand on match point at 6-5 in the second set, but it was called long. Virginie Razzano of France rallied to win 3-6 7-6 (6) 6-1.

Slovakia's Daniela Hantuchova, seeded 17th, overcame Japan's Saori Obata 3-6, 6-3, 6-0 and No. 6 Nadia Petrova defeated Australia's Sophie Ferguson 6-2, 6-1.

No. 9 Elena Dementieva, a semifinalist at the last U.S. Open, was the first seeded player to fall, losing 7-5, 6-2 to Germany's Julia Schruff.

No. 24 Tatiana Golovin and No. 26 Ai Sugiyama soon followed Dementieva. Golovin lost 6-4, 4-6, 6-4 to Mara Santangelo, and Sugiyama went down 6-4, 6-3 to Conchita Martinez Granados of Spain.

On the men's side, No. 8 seed Gaston Gaudio, the 2004 French Open champion, was leading 6-2, 5-0 when Romania's Razvan Sabau retired with an injured arm.

No. 13 Robby Ginepri only needed 1 hour, 19 minutes for a 6-1, 6-4, 6-2 win over Austria's Jurgen Melzer.

Also advancing were No. 7 Ivan Ljubicic, No. 11 David Ferrer, No. 16 Tommy Robredo, No. 17 Radek Stepanek and No. 18 Mario Ancic.

James Blake, seeded 20th, downed Jose Acasuso of Argentina 6-2, 6-3, 4-6, 7-6 (4).

No. 27 Taylor Dent lost in straight sets to Spain's Guillermo Garcia Lopez and former No. 1-ranked Carlos Moya went down to Andrei Pavel.

In the last night match, men's second seed Andy Roddick faced Switzerland's Michael Lammer.