Venus Williams sent out a warning on Monday as she outshone some of her biggest rivals on day one of the Australian Open.

Williams overcame a sluggish start to overpower Italy's Sara Errani 6-3, 6-2, as top seed Caroline Wozniacki, former winners Maria Sharapova and Justine Henin, and French Open champion Francesca Schiavone all struggled to impress.

Wozniacki took 1 hr 40 min to see off Gisela Dulko 6-3, 6-4, Sharapova struggled with her serve before beating Tamarine Tanasugarn 6-1, 6-3, and Henin had to pull out all stops to beat Indian qualifier Sania Mirza 5-7, 6-3, 6-1.

Meanwhile, French Open champion Schiavone lost the first set to Arantxa Parra Santonja before eventually winning 6-7 (4), 6-2, 6-4.

Williams, the United States' best hope in the absence of her injured sister Serena, the current champion, risks being underdone after not playing a WTA tournament since the US Open.

"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 warmed up with an exhibition tournament in Hong Kong. "I worked as hard as I could in the off-season - I worked hard to be here."

Williams has been something of an under-achiever at the Australian Open, reaching the final once, in 2003, but often falling early to unheralded opponents.

She lost in the 2010 quarterfinals to China's Li Na and went down in the second round in 2009 to Carla Suarez Navarro of Spain.

Earlier Wozniacki, under pressure to claim a maiden Slam title to convince critics she is a deserving women's No 1, labored to subdue 52nd-ranked Dulko, who made 38 unforced errors.

And former champion Sharapova admitted she had room for improvement after her win over 33-year-old Tamarine.

Sharapova has already bettered last year's performance, when she was shocked in the first round by fellow Russian Maria Kirilenko.

"I was definitely a little bit nervous in the beginning," Sharapova said.

"Last year I played the first match on center court and I lost, so I was like, I don't want this to happen again this year."

Henin, who won the Australian Open in 2004 and finished runner-up in 2006 and 2010, needed all her legendary fighting qualities to subdue an inspired Mirza in 2 hr 12 mins in a torrid first-round clash.

"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."

Li, who reached last year's semifinals and is fresh from victory at the Sydney International, went through against Sofia Arvidsson of Sweden 6-1, 7-5, while eighth seeded Belarusian Victoria Azarenka thrashed German Kathrin Woerle 6-0, 6-2.

Other seeded players to progress were France's 15th ranked Marion Bartoli, who trounced Italian Tathiana Garbin 6-0, 6-0, Estonian Kaia Kinepi (20), who beat Magdalena Rybarikova 2-6, 6-4, 6-3 and Slovakia's Dominika Cibulkova (29), a 6-2, 6-7 (4) 6-4 winner over German Angelique Kerber.

Belgian's 21st ranked Yanina Wickmayer ended the hopes of Australian Jarmila Groth 6-3, 2-6, 6-4.

There were two minor upsets, 17th seeded Frenchwoman Aravane Rezai going down to Czech Barbora Zahlavova Strycova 6-0, 3-6, 7-5 and Russia's Regina Kulikova downing 28th ranked Slovakian Daniela Hantuchova 7-6 (3), 3-6, 9-7.