A cramping Carlos Alcaraz had to draw on every physical and mental reserve to come back from the brink and beat Alexander Zverev to reach a first Australian Open final.
Spain's Alcaraz came through 6-4 7-6 (7-5) 6-7 (3-7) 6-7 (4-7) 7-5 after an enthralling five hours and 27 minutes - but the scoreline barely hints at the drama that occurred in Melbourne.
Alcaraz had clinched the first two sets and was cruising when, serving at 4-4 in the third, he suddenly pulled up and was left barely able to serve or move.
The world number one took a medical timeout - leaving third seed Zverev irate - and looked a shadow of his former self as his German opponent forced a decider.
Zverev went an early break up in the fifth set and served for the match at 5-4, before Alcaraz produced a scarcely believable comeback.
Buoyed by the crowd, Alcaraz broke to get the match back on serve and then capitalised on Zverev's nerves as the German served at 6-5 down to try to force a 10-point tie-break.
Alcaraz fell to the floor in complete disbelief as Zverev missed a backhand and sent him into Sunday's showpiece.
Victory keeps alive the 22-year-old's bid to write more tennis history - should he win on Sunday, he will become the youngest man to complete the career Grand Slam.
Only Novak Djokovic - and Alcaraz's own physical condition - now stand in the way of a fourth successive Grand Slam final between the Spaniard and Jannik Sinner.
Serb Djokovic, bidding for a record 25th major singles title, faces world number two Sinner later on Friday.
Alcaraz's bravery pays off once again
Alcaraz has produced some astonishing comebacks at Slams - none more than when he saved three championship points before going on to beat Sinner in last year's French Open final.
But this win, given his physical condition, will be just as pleasing.
Alcaraz stood at the back of court, telling himself "little by little, Charly, vamos" as he tried to find some energy late in the fourth set.
Even before he began cramping, Alcaraz was the braver player. He took control of the rallies, going for the big shots - and finishing the match with 78 winners to Zverev's 56 showed how his 'big-time' tennis paid off.
Zverev played at a high level, but was not ruthless enough when it mattered.
Alcaraz could barely at the end of the third set, but Zverev sprayed errors off the weakened serve and rarely tested him at the net.
Twice Zverev had 0-30 on the Alcaraz serve to force a fifth set. Twice Alcaraz, urged on by the crowd, held, and then served to love to send the fourth set to a tie-break.
However, Zverev took advantage of Alcaraz's faltering serve in the tie-break and again in the first game of the fifth set to go an immediate break up.
But the German has crumbled in key moments before and his break lead never felt safe.
Alcaraz put him under immediate pressure as he served for the match, and Zverev was ultimately left to contemplate another devastating loss.
What are the rules on cramping?
Alexander Zverev vented to a tournament official during Carlos Alcaraz's first medical timeout
Zverev berated an official when Alcaraz took a medical timeout in the third set, calling it "unbelievable".
Alcaraz was 5-4 up when he took the timeout. Within his opponent's earshot, Zverev repeatedly said that the Spaniard was cramping and not injured, and therefore should not receive any treatment.
Alcaraz had treatment to both thighs and was also drinking pickle juice, but his movement improved as the match went on.
Players can take a three-minute medical timeout for physical injuries or illness but not for cramping, as it is seen as a conditioning problem rather than a physical injury.
They can only be treated for muscle cramping during either a change of ends or the end of a set breaks.
If a player cannot continue because of severe cramping, they may forfeit a point or a game to force a change of ends and receive treatment.
However, if they claim to have an injury but the physiotherapist or doctor decides they have cramp, they will be told by the chair umpire to immediately resume playing.
It is not the first time Alcaraz has struggled with cramp in a Grand Slam semi-final.
Stress led to the then-20-year-old being hit by full body cramps against Novak Djokovic at the 2023 French Open after two tight sets, and he quickly faded.
When Alcaraz cramped against Djokovic two years ago, the Serb exploited his lack of movement, and he could do little more than see out the match.
Zverev was not as ruthless - and he was ultimately made to pay for it.
BBC