Nadal's health among X-factors in Oz

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Roger Federer was twice in a position to win four successive majors. Both times he was denied by Rafael Nadal.

Nadal is the one now on the verge of claiming four in a row, and wouldn't it be ironic if Federer derails his younger rival at the Australian Open?

The absence of two-time defending champion Serena Williams means the women's draw is more open than usual.

ESPN.com poses five questions heading into 2011's first major, which begins Sunday in the U.S.

How healthy will Nadal be?

The knees don't seem to be a problem for Nadal as he tries to emulate Rod Laver, the last man -- and only one in the Open era -- to capture four in succession, 41 years ago.

Nadal's schedule has been debated in the past, and why the Spaniard took only "three or four" days off following a mentally draining season is beyond many. He needed more than that.

Nadal subsequently developed a fever in Qatar -- after winning a lucrative exhibition in Abu Dhabi -- and had to delay his journey into Melbourne. Not a good sign, even for a physical specimen such as the 24-year-old.

It's best of five sets, but having days off between matches is the good news for Nadal.

Can Federer keep it up?

Federer, meanwhile, has always done a good job of handling his schedule; last year, he suggested the forthcoming extended offseason wouldn't affect him much because he plans for breaks during the campaign.

Federer recharged following a busy -- and successful -- fall, picking up where he left off by claiming a third title in Doha.

On serve, anyway, the Swiss appears to have returned to his best, which means the back is fine. Only once was Federer broken in Qatar, and against one of the best returners in the world -- Nikolay Davydenko -- he didn't face a break point in the finale.

During his 26-2 run in his past 28 matches, Federer has been broken a combined 20 times, and four of those came in one match against Andy Murray in Shanghai. Federer beat Murray when it counted, in last year's Australian Open final, for a 16th major.

Ominous stats for the rest of the field.

Will Clijsters win outside New York?

On the one hand, Kim Clijsters says this year might be her last full season. Then she says she could keep going for four more years.

Now, the two can co-exist, since Clijsters could limit tournaments in 2012, 2013 and 2014, but you get the feeling that once the Belgian starts thinking about that, she'll stop altogether. Clijsters and hubby Brian Lynch want a sibling for bubbly Jada.

So Clijsters had better collect majors while she can.

She is the favorite by some margin in Melbourne, given Serena's no-show, and she proved she could deal with the pressure last summer as the defending champion at Flushing Meadows.

Left soured by a humiliating loss -- 6-0, 6-1 -- to Nadia Petrova in the third round in 2010, Clijsters surely wants to make amends and walk away with a first Grand Slam title outside the U.S.

What about Juan Martin del Potro?

Del Potro's 2010 was a washout due to a serious wrist injury -- the former U.S. Open champion turned up at only three tournaments.
He's back and vows the wrist is OK, although his own schedule heading into the Australian Open is slightly puzzling. Needing matches, the Argentinean chose to compete at one warm-up, not two, and that was this week in Sydney.

If he wanted to stick to one prep tourney, why not compete last week, then practice at Melbourne Park? Who knows how his body will hold up if he has an extended stay in Sydney. Del Potro had an extended first-rounder, saving a match point in a thrilling 6-7 (5), 7-6 (9), 7-6 (3) victory over lefty Feliciano Lopez in three hours, 20 minutes.

Del Potro would probably consider it a success to reach the second week. Getting in gear for the remaining three majors must be the priority, aside from staying healthy.

Does Henin have a chance?

Away from the game for most of the previous 20 months, Justine Henin downplayed her chances at the 2010 Australian Open.

What a kidder, that Henin.

She battled her way to the final, falling short -- barely -- against Serena Williams.

Not surprisingly, Henin is again minimizing her chances, returning from what she says was a career-threatening elbow injury that still isn't 100 percent.

Henin, mind you, looked sharp at the Hopman Cup in Perth, not losing in singles, and there's no Serena to contend with.

Of course she has an opportunity. A sizable one.