OF all the things the Swiss has accomplished in a career that is into its 21st year, including his seven Wimbledon titles and 17 Grand Slam crowns in all, perhaps Roger Federer's achievements over the past 11 days here at the Australian Open may eventually top the lot.

At 35, and having missed six months of tennis in the second half of 2016, Federer arrived in Australia hopeful but unsure how exactly his knee would respond to the rigours of the Tour and the stresses of competition.

Yesterday, he outlasted his fellow Swiss Stan Wawrinka in five sets, mixing glorious shot-making with the inevitable tension of a Grand Slam semi-final and, on Sunday, he will battle for an 18th Major crown against either Rafael Nadal or Grigor Dimitrov, who contest the second semi-final today.

“I never, ever would have thought I could go this far,” Federer said, after his 7-5 6-3 1-6 4-6 6-3 win over the 31-year-old Wawrinka, who was left to rue a couple of break-point opportunities at the start of the final set, when he seemed to have turned the match around.

Four and a half years on from his last Slam triumph, at Wimbledon, Federer struggled for his usual fluency in his opening two matches.

Everything turned in a stunning display as he swept aside No.10 seed Tomas Berdych in round three, since when he has beaten fifth seed Kei Nishikori, Andy Murray’s conqueror Mischa Zverev and then, most impressively of all, Wawrinka.

The 31-year-old has won three Grand Slam titles, one per year since 2014, including the US Open last September, and he has built a reputation for playing his best tennis in the second week of Major tournaments.

Federer took the opening set thanks to one break late on and then upped his game in the second set, after which Wawrinka took a medical time-out for treatment on his right knee.

When he returned, he relaxed and began to start ripping winners from the baseline, storming through the third set and then taking the fourth to force a decider.

At that point, Federer took a medical time-out himself, admitting later that he has been suffering from an issue with his leg throughout the event, albeit one that will not affect him in the final.

“I think these injury time-outs are more mental than anything else,” Federer said. “If you have something with your groin or something like that, you need tape and have to go off court, it’s the first time during a match we can talk to someone. That can be positive.”

Wawrinka had a break-point at 1-1 but pushed a short backhand just wide and then at 2-2 he netted a routine backhand. At 2-3, Wawrinka played two loose points and then double-faulted to hand Federer a break he would not relinquish.

“Midway through the fourth when I realised my game was fading, Stan was having the upper hand on the baseline and I thought, I guess that's what I was always talking about,” he said. “Things turn for the worse, you don't know why.

“But the good thing is, I did have the cushion from the first two sets. I think I did a lot of things right. I prepared the match in a way that allowed me to win it later on.”

From 4-2, Federer never looked back, serving out comfortably to reach his sixth Australian Open final.

“I'll leave all the energy here in Australia, and then I can relax after here,” he said. “It's gone much better than I thought it would. That's also what I was telling myself in the fifth set. I was talking to myself, saying like, 'Just relax, man. The comeback is so great already. Let it fly off your racquet and just see what happens'.

“I think that's the mindset I've got to have, as well, in the finals, sort of a nothing-to-lose mentality. It's been nice these last six matches to have that mentality. It worked very well so I'll keep that up."

For Wawrinka, who won the title in 2014, it was a disappointing end to a good fortnight.

“For sure I'm really sad and disappointed with a loss like that because to be that close to have won a semi-final, it can be only sad,” he said. “But at the end I know I tried everything on the court. I came from two sets down. I changed completely the momentum. I started to be extra aggressive because I had to change few things also physically.

“At the end I had a great battle against Roger. He's a great fighter. He's always been amazing in Grand Slams, in five-set matches. I'm for sure sad to lose a match like that but I know there are a lot of positives.”

Federer said he had no preference who he plays in the final, even if he has never lost to Dimitrov and trails Nadal 23-11.

“You would probably think I would have a slightly better chance to beat Dimitrov than Nadal, but who cares?” he said. “It matters if you win or not. I am in the final, I know that, I know I’ll be there on Sunday, that’s a special position to be. All I care about is can I win the match.”