Roger Federer beat Stan Wawrinka in a stunning five-set match
Wawrinka slipped two sets down and looked out of the match when he left the court in tears to receive treatment on a troublesome knee.
But the medical timeout appeared to invigorate him as the world No 4 won the next two sets to force a decisive fifth.
And Roger Federer, who also took a medical timeout, served out the final set to complete the first half of a potential final meeting with Rafael Nadal, which would be the 35th meeting of the pair.
The match between Roger Federer and Stan Wawrinka could have gone either way
The Spaniard faces Grigor Dimitrov tomorrow in the second semi-final.
Federer came into the match having spent the second half of the 2016 recuperating from a knee injury.
However, his form at the Australian Open has been exemplary, cruising past Andy Murray's conquerer Mischa Zverev in the quarter-final in straight sets.
Stan Wawrinka aggravated a knee injury but appeared to come out of it refreshed
And it looked as though the Swiss might repeat his feat of comfort when he took the opening set 7-5.
The two friends were producing some sensational tennis but Federer was smothering his opponent's game – Wawrinka failed to record a single backhand winner in the second as the former world No 1 converted his second break point to take it 6-3.
Two sets down, Wawrinka left the court for a medical timeout with tears in his eyes and a towel over his head.
When he returned with some minor strapping on his right knee, his movement did indeed seem more limited.
However, his tennis seemed to benefit.
Wawrinka broke Federer twice in third set to win 6-1, allowing himself to hit massive baseline winner after winner and his opponent's serve suffered.
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The momentum continued into the fourth set as Wawrinka broke in the first game but immediately dropped his serve again.
However, he took the third break point in ninth game of the set and subsequently served out to level the match.
Both players seemed tight, Federer playing for a sixth Australian Open final and Wawrinka his second.
But the most crucial game seemed to be the fifth. Federer saved a break point and went to deuce three times before holding.
In the aftermath, he broke quickly and served out to love three games later to move within one win of an 18th Grand Slam title after a little over three hours.