Roger Federer beat fellow Swiss Stan Wawrinka 7-5 6-3 1-6 4-6 6-3 to reach the Australian Open final and stay on course for an 18th Grand Slam title.

The 35-year-old will face Rafael Nadal on Sunday if the Spaniard beats Grigor Dimitrov in Friday's semi-final.

The Swiss, returning from a six-month lay-off to rest his left knee, last won a major at Wimbledon in 2012.

He is the oldest man to reach a Grand Slam final since Ken Rosewall did so at the 1974 US Open at the age of 39.

"I couldn't be happier right now," said Federer. "I felt like everything happened so quickly at the end, I had to check the score.

"I never ever in my wildest dreams thought I'd come this far in Australia. It's beautiful, I'm so happy."

Federer's extraordinary run in Melbourne had already seen him beat higher seeds Tomas Berdych and Kei Nishikori to reach the last four.

Seeded 17th following his injury, Federer had an 18-3 record against the fourth seed and reigning US Open champion coming into the semi-final, but the two had never played a five-set match.

Both players needed medical treatment during a match of high intensity but it was the 17-time Grand Slam winner who finally prevailed after three hours and five minutes.

Federer will now seek a fifth Australian Open title, and his first since in Melbourne since 2010, when he plays in his 28th Grand Slam final on Sunday.

Federer dominates before Wawrinka rallies

Former world number one Federer started the match brightly and had three early break points before converting his first set point, on Wawrinka's serve, in the 12th game.

Wawrinka, the 2014 Australian Open champion, was broken for the second time at 2-3 in the second set as Federer maintained his impressive standards.

Clearly frustrated, the 31-year-old Wawrinka cracked his racquet in two over his left knee and, after the set, left the court with a trainer for treatment to his other knee.

But he came back superbly to win the third set in 26 minutes and break Federer in the ninth game of the fourth set to take the match to a decider.

Federer went off for a lengthy medical timeout for treatment to his leg as the physical nature of the match started to tell.

But he came back fighting and broke Wawrinka in the sixth game when the US Open champion double-faulted on break point.

There was no let-up as Federer completed a stunning victory to the delight of the majority of fans in Rod Laver Arena.

Physio's magic hands help Federer

Federer explained why, after losing the fourth set, he left the arena to take his injury timeout.

"I have had a leg thing going on for a week, and felt it from the second game on in the match," he said.

"If you go off the court, that means the treatment is further up the leg.

"I never take injury timeouts. Stan took his, so I thought people won't be mad - Stan won't be mad hopefully.

"You hope something works, and that physio has some magic hands going on."