Roger Federer battled back from the brink to clinch a dramatic five-set comeback against Tennys Sandgren and reach the Australian Open semi-final.
The 38-year-old breezed through the first set but was stunned when the American stormed back to win the second, and Federer began to struggle.
The Swiss veteran appeared to be carrying an injury and went off court for treatment after losing the third set, although it was unclear what injury he was supposedly carrying.
He rallied on in the fourth set to take it to a tiebreak and saved an incredible seven match points in total before forcing a deciding set.
After breaking in the sixth game for the first time since the first set, Federer - against all the odds - roared to a 6-3, 2-6, 2-6, 7-6, 6-3 victory against the 28-year-old.
Federer will make his 15th semi-final appearance in Melbourne, extending his own record, and will face either Novak Djokovic or Milos Raonic in the final four.
For a player ranked 100th in the world, Sandgren had shown that his ranking was misleading at best, having already beaten top-15 seeds Matteo Berrettini and Fabio Fognini on his way to the quarter-finals.
But facing Federer was a different proposition altogether and so it proved as the Swiss stormed to a double break inside 30 minutes.
Sandgren found his rhythm in the second set, racing into an early break and sealing the set 6-2 before repeating the same in the third set.
Federer was visibly struggling with his movement and went off court for treatment, returning after a five-minute delay to play out the fourth. He battled hard, saving three match points and refused to give an inch on his serve and took Sandgren to a tiebreak.
When the American went 6-3 up, with three more match points, the game looked up for Federer. But he continued to fight, rally and dig out some incredible shots to save another three match points.
The tiebreak went on until Federer got his set point and when Sandgren played his overhead long, the crowd roared in delight. Federer had forced the final set, somehow.
The seriousness of the injury appeared to subside in the final set as Federer found his momentum on serve and eventually made Sandgren pay for some costly misses to break in the sixth.
And after a gruelling three and a half hour duel at the Rod Laver Arena, Federer wrapped up the most remarkable of victories to continue his fight for a 21st Grand Slam, while Sandgren left scratching his head.
