MELBOURNE, Australia -- Serena and Venus Williams continued their astonishing run in doubles matches with comfortable victories in the first two rounds.

The sisters cruised into the third round with a 6-3, 6-2 victory against Casey Dellacqua and Nicole Sewell after Australian Open organizers broke with convention and named them as top seeds.

The American pair, who have won the doubles title at all four grand slams, have not been beaten since the 2001 U.S. Open but still needed a wild card to get into the Australian Open because Venus does not have a doubles ranking.

Under WTA rules, players need to compete in at least three events to earn a ranking but Venus played just one last year, Wimbledon, which she and Serena won.

Having awarded the duo a wild card, Australian Open officials named the Williams sisters as the top seeds to avoid them running into the best teams too early.

There were complaints when the sisters won the Australian Open doubles in 2001 unseeded after they beat most of their top rivals before the final.

Serena said she was surprised to receive top billing but thought it was fair.

"I was shocked and stunned. I didn't understand it at first. Venus told me but I just didn't believe her," Serena said.

"From what I understand, because we're a wild card, it will be hard for us to play a top seed. That would be a tough match for either of us.

"I think it's easier for us to be seeded, so the top players won't play us in the first round."