TENNIS/Australian Open Championships: The Williams sisters have never been particularly gracious losers, often preferring to make excuses rather than acknowledge that an opponent has played better.

So it was hardly surprising that when Serena was asked whether she needed to win the Australian Open to counter talk of any decline, she replied sternly: "I don't appreciate that language."

She had just defeated France's Amelie Mauresmo 6-2, 6-2 with considerable ease to reach tomorrow's semi-finals where she will play the new Russian number one Maria Sharapova, who beat Williams in last year's Wimbledon final and the final of the season-ending championships in Los Angeles. It will be a critical match for Williams.

"I don't have to win this tournament to prove anything," she insisted. But, of course, she does, because it is the nature of sport to compare like with like.

For various reasons, some of them due to injury, others concerned with striking a balance between on and off court activities, Serena and Venus have slid down the tennis scale.

A few years ago their rivals were few, and largely intimidated by the Americans' immense power and athleticism. Today's youngsters have no such inhibitions; they hit the ball every bit as hard and many are technically superior, including Sharapova.

However, the 17-year-old Wimbledon champion had to put up an immense fight before defeating her fellow Russian, and the reigning US Open champion, Svetlana Kuznetsova, and it is possible that the combination of physical exertion and the acute heat will exact a payback when she meets Williams.

Kuznetsova, stocky and muscular, dominated the opening set, with her forehand frequently accelerating beyond Sharapova's extensive reach. Yet despite her early dominance in the rallies, her first serve was malfunctioning.

Sensing the weakness, Sharapova set about exploiting it with savage intent. She broke back immediately, and thereafter Kuznetsova was unable to hold her serve again, losing 4-6, 6-2, 6-2.

From having all the enthusiastic bounce of a young puppy, the 19-year-old Kuznetsova sunk into ever increasing despair. "It's all in my mind. I have the game. I have everything. But if you don't do the right things, you don't win."

Providing she recovers, Sharapova, whose will to win borders on frightening, seems certain to push Williams to her limits. The six-time grand slam winner still believes she is the best in world, but the proof has been missing for nearly a year and a half at grand slam level.

Tomorrow will be an acid test for Williams. Failure may mean an accelerated slide.