Defending champion Serena Williams warned her rivals she can get even better despite playing brilliant tennis to crush Samantha Stosur 6-4, 6-2 in the fourth round of the Australian Open on Monday.

Williams wiped the big serving Stosur off the court in just 65 minutes to reach the quarterfinals, where she will play either Victoria Azarenka of Belarus or Russia's Vera Zvonareva.

She remains on track for a semifinal against sister Venus and a possible blockbuster final against Belgium's Justine Henin.

The world No 1 gave herself only a B plus when asked to rate her game, saying perhaps she could have returned better.

"I think Sam really burned me on a couple of returns but she's serving incredible," Williams said.

"It's really good to match my game against her serve because she's one of the best servers on the tour."

By rating her game only a B plus, Williams was being tough on herself as the four-time champion never looked in any trouble against the Australian No 1.

She served brilliantly throughout both sets and dismantled Stosur's serve as the match wore on, quickly silencing the large partisan crowd.

"It's important when you're playing a local girl to not let the crowd get too involved or else they'll kill you," Williams said.

"That was the plan - to not let them get involved."

Stosur had gone into the fourth-round clash on Rod Laver Arena with some hopes of an upset after beating Williams the last time the pair met.

Stosur had also complained that Williams had not given her much credit for that win, saying the American implied she was a lucky player, with many of her winners coming from mishits.

But yesterday she came up against a fiercly determined Williams and had no answers.

Williams conceded just two points on serve in the first set and three in the second to completely demoralize Stosur.

"I think today it was all about the serve," Williams said.

"Honestly, I'm just hoping I can serve again like that because that was pretty cool.

"I knew going into the match I had to serve well against Sam. She's a big server and even though she's not as tall, she serves incredibly well."

Despite saying she could have returned better, the world No 1 was almost as ruthless as when she was serving, breaking Stosur once in the first set and twice in the second to wrap up a comfortable win.

"When someone's playing that aggressively and not giving you anything at all, she's pretty hard to beat," said Stosur.