Aus Open: Serena Williams in quarter-finals

Serena Williams had a fortunate passage into the Australian Open quarter-finals today when Victoria Azarenka retired with illness while leading 6-3, 2-4 in their fourth round match.

Azarenka, the 2005 Australian junior champion from Belarus, had broken Williams twice to win the first set and was down a break in the second when she appeared to become distressed.

She called for a medical time out to get attention at 2-3 in the second set, and although she came back out to play she could barely stand up between points and eventually walked to the net in tears to concede the match.

Azarenka had looked like causing yet another huge shock at this year's Open when she dominated Williams, a three-time champion, in the first set.

Williams had started well by holding her first service game comfortably then bringing up break points in the next game, but that was as good as it got for the second seed in the first set.

Azarenka landed 82 percent of her first serves and was never in any danger of losing her service games, whereas Williams struggled to hold every time.

The Belarusian broke Williams in the seventh game and again in the ninth to take the first set comfortably. Williams was so frustrated with her serve she swore at herself in anger during the ninth game and was warned by the umpire for an audible obscenity.

Azarenka kept up the pressure in the second set and had break points to go to a 2-0 lead but Williams held on, then out of the blue broke the Belarusian for the first time in the match.

Azarenka got to 15-40 on Williams' next serve but the American finally found some serving form and dug herself out of trouble.

The drama then began when Azarenka appeared to become ill at the change of ends and called for a medical time out, leaving the court with a trainer.

She came back out for the next game, won by Williams, but on her next service game she became more and more distressed and was staggering between points.

She somehow got to 30-0 but when Williams got back to 30-30 Azarenka realised enough was enough and the second seed was through to a quarter-final against Svetlana Kuznetsova.