Melbourne - Roger Federer's Grand Slam renaissance faces the difficult if familiar obstacle of Stan Wawrinka on Thursday in an all-Swiss blockbuster for a place in the Australian Open final.
The 35-year-old Swiss marvel has been the talk of the year's first major in his unforeseen, roll-back-the-years charge at an 18th Grand Slam crown.
Federer has surprised even himself in reaching the semi-final after missing the second half of last year following knee surgery, with his ranking slipping to 17.
He has tossed aside top-10 players Tomas Berdych and Kei Nishikori, along with Andy Murray's conqueror Mischa Zverev, to reach the last four and a date with long-time Swiss team-mate Wawrinka, the world number four.
"If someone would have told me I'd play in the semis against Stan, never would I have called that one for me. For Stan, yes, but not for me," Federer admitted.
It is an intriguing match-up as the all-time great sizes up his long-term accomplice Wawrinka, with much on the line.
The statistics point to a Federer victory, after 18 wins to just three in their career meetings and 5-1 in Grand Slam encounters.
But Wawrinka has developed into a great player in his own right under the coaching of Swede Magnus Norman, with three major victories in the last three years including a 2014 breakthrough triumph in Australia.
"He's become such a good player, I super-respect that, that the guy is able to transform his game in his footwork, in his mind, also in his game plan," Federer said.
"He's always been a believer that he can do it. It shows how far you can go."
Wawrinka, for so long in the shadow of Federer's stellar career, remains respectful but there is a self-belief that makes him a formidable opponent in Thursday's showdown.
"I'm more confident with myself. When I step on the court it doesn't matter who I play. I know what I have to do if I want to win," Wawrinka said.
"For sure, against Roger, it's always special because he's so good.
"He's the best player of all time. He has an answer for everything. But I managed to beat him in a Grand Slam, so we'll see.
"Most important is that I step on the court and I play my best tennis."
Wawrinka got one over Federer on clay in the quarter-finals of the 2015 French Open, although he was well beaten in their last Grand Slam meeting in the semi-finals of the 2015 US Open.
"Last time I got killed in US Open. He was playing way better than me, moving really well, really aggressive from the court," he said.
"He's playing so well since the beginning of the tournament. He's really flying on the court.
"It's great to see him back at that level. Hopefully I can manage to play a great match."
In the recent dominance of Novak Djokovic and Andy Murray, Federer's hopes of adding to his record 17 Grand Slams appeared forlorn, given his advancing years.
Federer is looking to become the second oldest man in the Open era after Ken Rosewall (at 37) to win a Grand Slam title.
The Swiss legend won his last Grand Slam at Wimbledon in 2012 and made his last major final at the 2015 US Open.
But Murray and Djokovic are no longer in Melbourne after stunning early round exits, boosting Federer's initially remote chances.
It is a fascinating sub-plot that his close friend Wawrinka may yet have the final say.