Australian Open, Melbourne
Venue: Melbourne Park Dates: 17-30 January
Coverage: Watch on BBC TV, Red Button, BBC Sport website (UK only) & Eurosport; listen on BBC 5 live sports extra and online; text commentary online;
Wozniacki is still seaching for her first Grand Slam title
Top seed Caroline Wozniacki overcame a nervy performance to get her Australian Open campaign under way with victory over Argentina's Gisela Dulko.
The 20-year-old Dane, playing her first Grand Slam event as world number one, won 6-3 6-4 in one hour and 41 minutes.
Fourth seed Venus Williams was worked hard by Sara Errani before prevailing 6-3 6-2 and 14th seed Maria Sharapova beat Tamarine Tanasugarn 6-1 6-3.
Justine Henin, champion in 2004, saw off India's Sania Mirza 5-7 6-3 6-1.
Wozniacki is yet to win a Grand Slam title but can retain top spot in the rankings by reaching the semi-finals at Melbourne Park.
Against Dulko, she converted her only break point to help seal the first set and went a break up in the second.
The Argentine rallied to level at 4-4 but was broken back immediately before Wozniacki served out to set up a second-round match against Vania Kingwho beat Tamira Paszek.
"I really love coming back here, it's such a great event," said Wozniacki. "Playing here on Rod Laver Arena is an amazing feeling.
"My dream when I was a little girl was to reach number one in world and that happened to me last year. But it's a new year, a new start and I'm looking forward to playing here."
Williams finished with an worse first serve percentage and unforced error count than Errani, but hit two times as many winners as the Italian to advance.
The 30-year-old, whose sister and defending champion Serena is absent through a foot injury, has not played a tournament since the US Open in September and initially seemed to be struggling for rhythm.
But she responded to losing her first service game with a break of her own and secured a decisive advantage in the eighth game of the set.
The 2003 runner-up's progress was briefly delayed by light rain but she soon wrapped up victory in the second and next plays Sandra Zahlavova, who beat fellow Czech Renata Voracova.
"It's only just a matter of believing that I can come out and play well even though I haven't played as much as my opponents," said Williams,
who missed the last two months of the 2010 season with a knee injury.
"I worked as hard as I could in the off-season -- I worked hard to be here."
For Sharapova, who missed the 2009 event with a shoulder injury and lost to Maria Kirilenko in round one last year, it was a first victory at Melbourne Park since lifting the trophy in 2008.
Williams lost to China's Li Na in the quarter-final in 2010
The former world number one dropped her opening service game to love, but then won seven straight games as she took the first set and broke at the start of the second.
Tanasugarn, 33, recovered the break to lead 3-1 and had a game point on serve for a 4-1 advantage, but Sharapova broke back and regained the momentum, winning the next five games and sealing her win with a fine backhand winner.
"I knew I had an early exit last year and didn't want it to happen this year," said the 23-year-old three-time Grand Slam champion.
"I really enjoy coming here and whenever you win at a place it brings back greater memories than other places."
Asked what she has been working on in the off-season, Sharapova replied: "Staying aggressive, trying to get my game better, maybe to where it was a few years ago and even better."
Sharapova will next play Virginie Razzano of France after she beat Elena Vesnina 6-4 3-6 6-3.
Henin, whose 2010 season came to a premature end after the rupture of a ligament in her right elbow against Kim Clijsters at Wimbledon, looked short of her best against Mirza.
The 28-year-old won all four of her singles matches en route to the final of the Hopman Cup with Belgium, but suffered four breaks of serve as the opening set slipped away.
Henin quashed any hopes of an upset though as she tightened up her previously-lax play and Mirza folded in 28 minutes in the decider.
A defeated finalist in 2010, Henin stated that she was glad to be tested so early on in her Melbourne Park campaign.
"I think I needed this kind of match exactly to really be into the tournament like I want to be and be at my best level," Henin said.
"I have the experience, but I need matches, I need rhythm, and I need fight, like tonight."
Elsewhere, sixth seed Francesca Schiavone survived a scare to beat Spain's Arantxa Parra Santonja 6-7 (4-7) 6-2 6-4.
The reigning French Open champion was broken when serving for the match at 5-3 in the decider but hit straight book a meeting with Canada's Rebecca Marino.
Chinese ninth seed Li Na enjoyed a far more comfortable passage as she saw off Sofia Arvidsson 6-1 7-5, whole 15th seed Marion Bartoli of France demolished Tathiana Garbin 6-0 6-0.
Eighth seed Victoria Azarena of Belarus polished off Kathrin Woerle 6-0 6-2 of Germany in little over an hour.
The tournament's number 28, Daniela Hantuchova, became the first women's seed to fall as she succumbed 7-6 (7-3) 3-6 9-7 to Russia's Regina Kulikova in a match that lasted three hours 39 minutes.
After overcoming the 2008 semi-finalist, Kulikova will take on Chanelle Scheppers in the second round after the South African saw off Karolina Sprem of Croatia 4-6 6-4 6-2.
France's Aravane Rezai, seeded 17th, also bowed out after failing to recover from a sluggish start in her clash with the Czech Republic's Barbora Zahlavova Strycova.
World number 63 Zahlavova Strycova made light of the 41 ranking places separating her from Rezai as she took the first set to love before battling through in the decider to win 6-0 3-6 7-5.
The Rod Laver Arena crowd were left disappointed as Belgium's Yanina Wickmayer, seeded 21st, rode out the challenge of Australia's Jarmila Groth 6-3 2-6 6-4.
Meanwhile 23rd seed Svetlana Kuznetsova of Russia swept aside the United States' Alison Riske 6-3 6-2.