Switzerland's Roger Federer produced another dramatic escape as he battled back from the jaws of defeat to set up a semi-final showdown with old foe Novak Djokovic.
Before the match the Swiss joked that he had never played tennis against Tennys, but Tennys Sandgren showed this was no laughing matter as he pushed the six-time Australian Open champion all the way.
Federer secured one of six break points to edge the first set 6-3 as the crowd began to feel sure they were witnessing another routine victory for the Swiss legend.
However, this was not to be the case as Sandgren muscled his way back into the match with some fine hitting to capitalise on some unusual Federer errors and take the second set 6-2.
The world number 100 then continued to impress in the third set as he overpowered the 38-year-old with some precise hitting.
The American broke twice to take the third set 6-2.
Staring down the barrel, Federer knew he had to stay with Sandgren as both players continued to delight the crowd with some outstanding shots.
The fourth set went to a tie-break as Federer came up with some clutch tennis and strong tactical service points to astonishingly save seven match points to leave his opponent flustered.
The former world number one came up with some outstanding tennis right when he needed it most as he seized the tie-breaker 10-8.
Into the fifth and deciding set, Sandgren's head dropped after he realised the magnitude of what he was about to achieve, and Federer took full advantage sensing defeat in the body language of his tiring opponent that came so close to defeating the greatest player of all time.
The Basel native secured the vital break and never looked back to seal a truly captivating 6-3, 2-6, 2-6, 7-6 (10-8), 6-3 victory to the delight of the Melbourne crowd.
This was not the first time at this year's Australian Open where Federer has been pushed to the limit before dipping into his box of tricks to pull off another miraculous victory.
Having needed five sets in his third-round clash to see off gutsy Australian John Millman, this was another energy sapping win for the veteran Swiss legend who continues to defy time and logic.
This victory also ensured Federer would be the oldest Australian Open semi-finalist since Ken Rosewall in 1977. A truly outstanding feat.
Federer's legion of fans will rejoice at another marvelous victory but questions will be answered of his title hopes after more critical energy reserves where used up on court.
And with a fully fit Novak Djokovic awaiting him in the semi-finals, will the excursions both physically and mentally have taken too much out of the Swiss to challenge the Serbian in their 50th ATP Tour encounter.
Having came so close to defeating his rival in last year's Wimbledon final when he himself held two match points, Federer fans will be hoping that the great man still has a 21st Grand Slam victory left in him after all that he has achieved.