Serena Williams of the US celebrates a point during her fourth-round match against Samantha Stosur of Australia on day eight of the Australian Open tennis tournament in Melbourne. Williams won 6-4, 6-4. (AFP)

MELBOURNE, January 25, 2010 (AFP) – Serena Williams stamped her authority on the Australian Open while older sister Venus was made to look distinctly beatable as the women's fourth round was completed on Monday.

Defending champion Serena crushed Australian hope Samantha Stosur 6-4, 6-2 on Rod Laver Arena, wiping the 13th seed off court in just 65 minutes.

She will now play Victoria Azarenka in the quarter-finals following the Belarusian's 4-6, 6-4, 6-0 win over ninth seeded Russian Vera Zvonareva.

The winner of that match will face either Venus or Chinese surprise packet Li Na in the semi-finals, but what initially looked like a straightforward win for Venus over Li may not be so easy after all.

Venus struggled to get past Italian 17th seed Francesca Schiavone 3-6, 6-2, 6-1, while Li stunned fourth seed Caroline Wozniacki of Denmark 6-4, 6-3.

The 27-year-old Li joined countrywoman Zheng Jie in the last eight with her win, the first time two Chinese players have ever reached that stage of a Grand Slam.

"There is nothing better," Li said with a broad grin. "I mean this is good for us, for both players to reach the quarter-finals."

Venus had appeared heading for another early exit when she could barely get the ball in against Schiavone in the first set.

But she woke up after that and took control, winning a tough match in a shade under two hours.

Venus, a seven-time Grand Slam winner, dug deep to overcome Schiavone as she works to win one of the few tournaments to elude her.

She is in her 11th Australian Open campaign but is yet to take the singles crown, in stark contrast to Serena, who is a four-time champion.

But Venus is not beating herself up about her comparative lack of success in Melbourne, insisting a major part of her game plan is staying positive.

She said she could break the drought this year.

"Yeah, I'm ready to go and I feel very good," said Williams, who has won five Wimbledons and two US Open titles.

"Obviously, I'm hitting the ball pretty good, especially against her today. She was really playing well.

"I'm comfortable with my game, absolutely."

Venus and Li will both be looking over their shoulders at Serena, who cruised past Stosur then warned her rivals she can get even better.

The world number one 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," Serena 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, Serena was being tough on herself as she never looked in any trouble against the Australian number one.

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

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

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

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

Serena returned well also, 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," admitted Stosur.