(CNN) -- Sixth seed and home favorite Samantha Stosur became the first high-profile casualty at the Australian Open on Tuesday, but five-time champion Serena Williams eased to victory on her return from injury.
Hopes were high that world No. 5 Stosur could become the first Australian woman to win the Melbourne grand slam in 31 years, but she crashed out at the first hurdle after a 7-6 (7-2) 6-3 defeat to 59th-ranked Romanian Sorana Cirstea.
Stosur claimed her first major title at September's U.S. Open, but won just one match in her two warm-up events this year as she admitted to struggling under the weight of expectation.
Her poor form continued as she hit 33 unforced errors against a 21-year-old opponent who lost in the second round in 2010 and 2011.
"Obviously I'm extremely disappointed. There's not any other word for it but total disappointment," the 27-year-old said.
"Sorana hung in there and kept going for it and eventually got better and better. I think she played a very, very good match. You have to give credit where credit's due. But I think maybe I didn't step up."
Stosur is the fourth consecutive grand slam champion to be eliminated inside the first two rounds of the following major, with Kim Clijsters, Li Na and Petra Kvitova suffering similar fates at the French Open, Wimbledon and U.S. Open respectively. Of that quartet, all but Clijsters had won their maiden slam.
Cirstea, who has only once reached the quarterfinals of a major, will next play Poland's 99th-ranked Urszula Radwanska -- who won 6-4 1-6 6-2 against American Alison Riske.
Williams, seeking a 14th grand slam title, had no such problems despite having to wait until after midnight local time to seal her 6-4 6-2 win over Austria's 45th-ranked Tamira Paszek.
"I'm definitely a night owl, I can stay up all night," the 12th seed said after her first match since suffering an ankle injury in the Brisbane Invitational. "I really don't like mornings, but I don't mind them -- during a tournament only, though."
The American's five wins in Melbourne is a record for a woman in the Open era, although she hasn't played there since lifting the title back in 2010.
"Thank you guys, I really appreciate you guys staying," Williams told the crowd who had stayed until the early hours of Wednesday morning. "I haven't been here in over a year so I was a little nervous today, so thank you for getting me through it."
Williams will play Czech world No. 49 Barbora Zahlavova Strycova in the second round following the 25-year-old's 6-0 6-4 success against France's Iryna Bremond.
Second seed Kvitova's 6-2 6-0 triumph over Russia's world No. 82 Vera Dushevina was similarly comprehensive, with the Wimbledon champion wrapping up victory in 61 minutes.
The 21-year-old Czech, who can usurp Caroline Wozniacki at the top of the world rankings in the next fortnight, faces Spain's Carla Suarez Navarro in round two.
Fourth seed Maria Sharapova was also in a hurry, beating 68th-ranked Argentine Gisela Dulko 6-0 6-1 in just 58 minutes.
The 2008 champion, a three-time grand slam winner, will face 144th-ranked American Jamie Hampton in the second round.
Russian seventh seed Vera Zvonareva battled past Romanian world No. 61 Alexandra Dulgheru, as last year's semifinalist won 7-6 (7-4) 6-7 (5-7) 6-3 to set up a clash with 51st-ranked Czech Lucie Hradecka.
Ninth seed Marion Bartoli beat fellow Frenchwoman Virginie Razzano 7-5 6-0 and will next face Australia's former world No. 4 Jelena Dokic, who won 6-2 6-1 against 196th-ranked Russian Anna Chakvetadze -- a quarterfinalist in 2007 on her way to a career-high No. 5.
German 14th seed Sabine Lisicki will play Israeli Shahar Peer in round two after a hard-fought 6-2 4-6 6-4 win over Switzerland's Stefaine Vogele.
The 34th-ranked Peer disappointed the home crowd by thrashing Australian world No. 181 Isabella Holland 6-2 6-0.
Former world No. 1 Ana Ivanovic, the 23rd seed, had a comfortable 6-0 6-3 defeat of Spain's Lourdes Dominguez Lino. Serbia's 2008 French Open champion will next play Dutch world No. 91 Michaella Krajicek.
Russian seeds Svetlana Kuznetsova, Anastasia Pavlyuchenkova and Maria Kirilenko also went through, with the latter to face Canada's Aleksandra Wozniak after eliminating Australia's Jarmila Gajdosova.
Slovakian 17th seed Dominika Cibulkova earned a clash with Hungarian world No. 92 Greta Arn, while Italian No. 23 Roberta Vinci will come up against China's unseeded Jie Zheng and 27th-ranked Estonian Kaia Kanepi faces Russia's Ekaterina Makarova.