Rafael Nadal and Roger Federer will go head to head in a vintage Australian Open final after the Spaniard sealed an epic five-set victory over Grigor Dimitrov.

Nadal triumphed 6-3 5-7 7-6 (7/5) 6-7 (4/7) 6-4 after just short of five hours on Rod Laver Arena and will now face Federer, his greatest rival, in a mouth-watering showdown on Sunday.

Dimitrov threatened to derail Melbourne's dream reunion by twice coming from behind to force a decider, but Nadal was relentless, powering his way through to his 21st major final and first since winning the French Open in 2014.

That he meets Federer there will be viewed as the perfect closing chapter to a throwback Australian Open, which now boasts four singles finalists aged 30 or older - a first for a grand slam in the Open era.

Venus Williams, 36, and her sister Serena, 35, will fight it out for the women's title on Saturday, in an exact replica of the finals bill at Wimbledon nine years ago.

Nadal then defeated Federer 9-7 in the fifth set, in what is widely considered the greatest match the sport has ever seen, but few would have predicted the pair would be meeting again for their ninth grand slam final, 34 major tournaments later.

Nadal will be chasing his 15th grand slam title to edge one closer to Federer, who stands on the brink of his 18th.

"It's a privilege," Nadal said on court afterwards. "It's a very special thing for both of us to be in the final of a major again and have the chance to compete against each other after a couple of years out with some problems.

Nadal collapsed to the floor in delight after seeing off his opponent (Getty)

"It was a moment we never thought we were ever going to have again, to be in the final in Australia, so we both feel very happy."

Nadal will also be bidding to win his second Australian Open title, which would see him become the first man in the Open era to win all four grand slam titles twice.

He was nearly undone here by Dimitrov, who came within a whisker of reaching his first major final and showed why many still consider him to be a multiple champion of the future.

The 25-year-old may well look back on three break points missed in the fifth set and wonder what might have been.

It was not so close at the start, as an angled volley sealed the first set for Nadal, who had made just two unforced errors and won 18 of 20 points on his first serve. Dimitrov could do little but wait.

The two players embrace after their epic semi-final clash (Getty)

His time came in the second, with Nadal serving to stay in the set at 5-4, as Dimitrov had four set points but struggled to get over the line.

Nadal saved the first with a lassoing forehand and the last with an ace but in between Dimitrov was too generous, as two unforced errors helped his opponent to hold.

If this was a test of character, Dimitrov passed as he came again two games later to finally grab the set.

At 6-5 in the third, with Nadal serving for a tie-break, there was a six-minute delay while a spectator required medical attention.

But when the action resumed, Nadal powered into the tie-break for a 4-2 lead, before clinching the set when two Dimitrov forehands missed their mark.

Neither player let up in the fourth, thrashing the ball from one end to the other in an awesome baseline battle, but no breaks meant again a tie-break would settle it.

This time Dimitrov moved 5-2 clear and then a cool backhand volley gave him three set points. Nadal saved one but not the next as a thundering serve sent the match to a decider.

Nadal plays a shot in the fifth set against Dimitrov (Getty)

Nadal raced out of the blocks, opening up three break points in the first game, but failed to convert, and then it was Dimitrov's turn, a stinging backhand down the line giving him 30-40 but Nadal also held on.

There were two more break points apiece before finally Dimitrov submitted at 4-4. Nadal whipped away a forehand pass, hunted down Dimitrov's retrieval and drove it down the line for the break.

Serving for the match, Nadal delivered an ace for match point, called out but shown to have nicked the line on review. Dimitrov saved one with an ice-cool smash and then another with a blistering forehand.

A third match point proved beyond him, however, as a weary backhand drifted past the baseline. Nadal fell to his knees in celebration after an enthralling four hours and 56 minutes.