Australian Open top seed Serena Williams showed she was back to her Grand Slam best Tuesday, as former world number ones Caroline Wozniacki and Victoria Azarenka set up an enticing second-round clash.

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Eighteen-time Grand Slam champion Williams shook off her erratic early season form to sweep into the second round at Melbourne Park, demolishing Alison Van Uytvanck of Belgium 6-0, 6-4.

Petra Kvitova, seeded four, Agnieszka Radwanska (six), Wozniacki (eight), last year's finalist Dominika Cibulkova (11), and Venus Williams (18) were among the other seeds to advance.

But the upsets that marked day one of the season-opening Grand Slam continued with first-round exits for former world number one Jelena Jankovic (15), Andrea Petkovic (13) and Flavia Pennetta (12).

Serena Williams, 33, is chasing a fifth Australian title that would take her Grand Slam tally to 19, placing her outright second on the all-time Open Era list behind Steffi Graf's 22.

The American could lose the top ranking she has held for 100 consecutive weeks if she falters in Melbourne but, with characteristic confidence made a "number one" gesture after her win, sending a message to her rivals.

She faces a potentially tricky second round tie against Russia's Vera Zvonareva, a former world number two with two career wins over Williams, who is returning from shoulder surgery.

But most attention in the second round will be on the mouth-watering showdown between Wozniacki and Azarenka, a two-time Australian champion unseeded this year after suffering major injury problems.

Wozniacki, who downed US teenager Taylor Townsend 7-6 (7-1), 6-2, said she and Azarenka were good friends on tour although the Belarussian was more focused on beating her rival.

"I know she's very dangerous and we always had some tough matches. I'd like to just focus on myself and what I can do to build my game and prepare," said the 25-year-old, who defeated America's Sloane Stephens 6-3, 6-2.

Kvitova ground out a 6-1, 6-4 win over Dutch qualifier Richel Hogenkamp but admitted she had to overcome a bout of nerves caused by memories of last year's humiliating first-round exit at the same venue.

Poland's Radwanska cruised past Kurumi Nara of Japan 6-3, 6-0 and Venus Williams dispatched Spain's Maria-Teresa Torro-Flor 6-2, 6-2, building on her win at the Auckland Open last week.