Roger Federer withstood a Marin Cilic comeback to successfully defend his Australian Open title and win a 20th grand slam crown.

Cilic recovered from two sets to one and a break down to force a fifth set but Federer proved stronger in the end to win 6-2 6-7 (5/7) 6-3 3-6 6-1 and join Novak Djokovic and Roy Emerson in claiming his sixth title in Melbourne.

"I'm so happy , it's unbelievable, it's been a long day," Federer said.

"I'm happy it's over now. The fairytale continues for us, for me, it's incredible," said an emotional Federer as he cradled the Norman Brookes trophy.

The 36-year-old, the second oldest man to win a slam title behind Ken Rosewall, moves four clear again of Rafael Nadal in terms of overall titles and is only three behind Serena Williams and four adrift of all-time record holder Margaret Court.

The crunch moment of the match came in the opening game of the decider.

Playing at his peak, Cilic overpowered Federer from the baseline but the second seed withstood the pressure to save two break points.

A successful HawkEye challenge on a Cilic second serve helped him break in the next game and another break for 5-1 left him serving for the title.

He needed only one match point, with Cilic netting a backhand return. The Croatian appealed to HawkEye but the serve just clipped the line.