By Nick Mulvenney

MELBOURNE (Reuters) - Roger Federer and Rafa Nadal take to Rod Laver Arena to reprise one of the great sporting rivalries of the modern age on Sunday, a fitting climax to an Australian Open which has sent tennis back to the future.

After the thirtysomething Williams sisters play their own throwback final on Saturday, Federer and Nadal will meet in a grand slam final for the first time in more than five years.

Their ninth meeting in a grand slam final is all the sweeter for being so unexpected.

Last October, as both men have recalled this week, Federer traveled to Mallorca for the opening of his friend's new tennis academy.

The Swiss was on "one leg" after knee surgery and the Spaniard still hampered by the wrist injury that wrecked his 2016 season.

"At that moment, for sure we never thought that we had the chance to be in a final again, and especially in the first of the year," recalled Nadal.

"Both of us I think worked very hard to be where we are. It's great that we are again in a moment like this."

As Nadal and Federer arrived at Melbourne Park as ninth and 17th seeds, therefore, the first drafts of the obituaries of two great sporting careers were being prepared.

They were hurriedly dropped back into the files, however, as it became apparent the duo were playing vintage tennis: Federer all fluidity and peerless shot-making, Nadal at his brutal best from the baseline.

The early departures of reigning champion Novak Djokovic and top seed Andy Murray, the players who had joined Federer and Nadal to make up the "Big Four", then opened up the path to a grand final reunion.

Although it has often felt more like 2007 than 2017 at Melbourne Park this week, even great champions are not immune to the ravages of time.

RECOVERY TIME

At 35, Swiss Federer is the oldest men's grand slam finalist since Ken Rosewall lost the U.S. Open final in 1974 at the age of 39.

Nadal might be five years younger but has been on tour since his late teens and the effort required for his attritional playing style puts tremendous strain on his body.

And while Federer has been able to rest up after his three-hour semi-final victory over Stan Wawrinka on Thursday, Nadal must recuperate as best he can after his epic five-hour victory over Grigor Dimitrov on Friday.

"That's what I'm going to try," the 14-times grand slam champion said. "I did it in 2009. I am seven, eight years older."

That five-set 2009 victory over Federer earned Nadal his sole Australian Open crown and another would make him the only man in the open era apart from Roy Emerson to have won all four grand slams twice.

Federer has four Australian Open titles but just a single French Open crown, his path to Roland Garros glory more often than not blocked by Nadal.

Immediately after his semi-final, as he was finally able to ponder meeting his great rival, the 17-times grand slam champion suggested his 11-23 record against Nadal might have been in part due to them meeting on clay so often early in their careers.

Afterwards, in his news conference, he was less candid, noting only that the pace of the Rod Laver Arena this year allowed him to be more aggressive.

"Now it's a different time," he said. "A lot of time has gone by. I know this court allows me to play a certain game against Rafa that I cannot do on center court at the French Open."

Nadal has a 6-2 edge over Federer in grand slam finals but few are making a call with any confidence over who will win Sunday's contest.

"I just know that two of the greatest players of tennis are going to square off on Sunday, and it's going to be a freakin' amazing match," Dimitrov said on Friday.

(Editing by John O'Brien)