Heat's on to find winner from this sizzling sextet as the Australian Open gets underway

This has always been the least predictable of Grand Slams because of the extreme heat of the Melbourne high summer that regularly has players (most memorably Pete Sampras) vomiting on the baseline. This year it is particularly hard to call because no one has much clue who the best player in either draw might be.

That the bookies make Roger Federer a warm favourite is pretty remarkable in itself a year after the final defeat that seemed to mark his terminal decline. That day he sobbed like one of the twin baby girls his wife was carrying after yielding to Rafael Nadal (Roger yielded to Rafa, that is, not Mirka; the paternity is not in doubt).

If the fifth set disintegration of his forehand and self-belief was painful to watch, his reaction to defeat was more so. The eruption of his lachrymals that foreshortened his runners-up speech ("Oh God, this is so hard," he wept) suggested the self pity of one facing the knowledge he would not win another major.

A few months later in Paris, however, he finally won the French Open, after runner-up Robin Soderling had taken out an ailing Nadal for him, to complete his Grand Slam collection. A month after that, he set a new record of 15 majors after that outlandish 16-14 final set Wimbledon triumph over the heroic, resurgent Andy Roddick.

And then, with Nadal wounded and Federer apparently poised to dominate once again, a new megastar was born in New York. Juan Martin del Potro, tennis's very own Lurch, used his flat forehand of barely credible speed - the most fearsome in tennis history - to bludgeon the Fed to another fifth set hiding.

Thus the Big Four became the Famous Five and, with Russia's Nikolay Davydenko playing sensationally to win the end of year ATP World Tour Finals in London, we now have a sextet of genuine Aussie Open contenders.

Perhaps the easiest to dismiss is Nadal, whose confidence has still to recover from that long absence with tendonitis-riven knees. The Mallorcan bull is the scheduled quarter-final opponent for Andy Murray, on whom the pressure to make the major breakthrough intensifies. He turns 23 in May and history teaches that those who go on to become tennis greats trouser their first major at no older than 22.

Having cancelled Christmas to stay in shape and prepare for the heat, he has a decent shot. He loves hard courts and, judging by his energypreserving demolition of a South African qualifier today, is in perfect condition. His challenge is to be relentlessly aggressive against more dangerous opponents because the policy of using subtle changes of spin and pace to tease mistakes isn't suited to winning Grand Slams.

At the highest level of any sport, exceedingly seldom does anyone win by trying not to lose. He has the power to dominate anyone. The question is whether he has the will to deploy it.

Novak Djokovic is back to his best after the regression that followed his victory here two years ago and is a fearsome threat. So is Del Potro.

Davydenko, a magnificent shotmaker when confident, could take his first major if his fair skin and bald pate handle the heat. The heart says Murray, the head says Djokovic, the pancreas favours Federer, the kidneys like Del Potro and the liver goes for Davydenko (although that may be the vodka talking there).

As for the ladies, for the unexpected intrigue of their tournament we must borrow from the lexicon of wartime cliche to thank Brave Little Belgium. Women's tennis had become alarmingly moribund before the returns of Kim Clijsters, maternal comeback queen of New York, and now Justine Henin, both first-round winners today.

The clock has been set back and this event looks like a straight fight between the Williams sisters' ferocity and the artistry of the Belgian blondes. Serena is the favourite and Clijsters the form horse but Henin may have the fiercer appetite to go with a one-handed backhand of such exquisite beauty that it belongs framed in the Louvre or Uffizi.

For all the imponderables, one thing may safely be predicted. The Aussie Open will throw up oodles of raw drama, thanks largely to the extreme heat that menaces our most sports-obsessed former dominion across the seas. "You cannot step in the same river twice", was another Heraclitus catchphrase but the old boy was wrong there. These days in drought-ravaged Australia, you'd be lucky to step in a river.