By Martyn Herman

LONDON (Reuters) - Rafael Nadal's rivals must pray he is still feeling the effects of flu at the Australian Open starts otherwise the Spaniard will be unbeatable in Melbourne, John McEnroe said on Wednesday.

However, he was practicing as normal in Australia this week and seven-times grand slam champion McEnroe is expecting the Spaniard to underline his current dominance despite dazzling early-season from by great rival Roger Federer.

"I would pick Nadal at the Australian because I think he's much tougher to beat in longer matches," McEnroe told the ATP Champions Tour website.

"If he remains healthy then he's the guy I would pick for every major right now until proven otherwise.

"I would love to see a guy like Juan Martin Del Potro or one of the other guys like Novak Djokovic or Andy Murray step up and really make an impact and win some majors.

Nadal, winner of the 2009 Australian Open, grabbed three of the four grand slams last year and with nine to his name, compared to the 16 of Federer, many are predicting he can one day eclipse the Swiss.

"The Australian Open is outdoors, it's in the heat and I believe Rafa has to be the favorite and probably will be the winner," Sweden's Stefan Edberg, twice Australian champions, said.

"It's incredible what he has done over the past year and he does have a chance now to catch Federer."

Fellow Swede Mats Wilander, a triple winner of the Australian Open, suggested Nadal was still improving.

"Nadal has improved more at number one than Federer did when he was there," he said.

"Federer has always played brilliant tennis but you never really saw him change, whereas Nadal has actually changed his game after getting beaten a couple of times last year. I would definitely pick him for the Australian Open."

(Editing by Ed Osmond)