Last year's runner-up Zheng Qinwen was knocked out of the Australian Open in the biggest shock of the tournament so far, while defending champion Aryna Sabalenka had to battle serve jitters to make the third round.

Fifth seed Zheng was sent packing 7-6, 6-3 at the John Cain Arena by World No. 97 Laura Siegemund of Germany, the second-oldest player in the draw at age 36.

Over on centre court Sabalenka dropped her serve three times and faced 11 break points before overcoming Spain's Jessica Bouzas Maneiro 6-3, 7-5, rattling off the last five games in a row.

"She played incredible tennis today and it was a really tough one. I expected this tennis from her, I'm really glad I was able to win this match," said Sabalenka.

The win kept the Belarusian World No. 1 on course for a rare hat-trick of consecutive Australian Open titles, a feat last achieved 26 years ago by Martina Hingis and only matched by four other women in history.

But Olympic champion Zheng's attempt to become only the second Chinese player to win a Grand Slam singles title after Li Na is over, after being out-thought by the powerful Siegemund.

"I knew I just had to play more than my best tennis. I had nothing to lose so I just told myself to swing free," said the German. (AFP)