In a men's draw decimated by shocks, never-say-die fifth seed Andrey Rublev saved two match points to topple Danish teenager Holger Rune in a five-set marathon.
In a battle of two former junior world No 1s, the Russian broke when Rune was serving for the match, then saved two match points before winning 6-3, 3-6, 6-3, 4-6, 7-6 (11/9) in sapping heat on Rod Laver Arena.
He will face either nine-time champion Novak Djokovic, who is struggling with a hamstring injury, or Australian hope Alex De Minaur for a place in the semifinals.
"It's not like a rollercoaster, it's like they put a gun to your head. A rollercoaster is easier," said the exhausted Russian of the 3hrs 37mins epic.
Also into the quarterfinals after another gruelling clash in the heat is Ben Shelton, the unseeded 20-year-old American who is on his first trip outside the United States.
His dream run at the Australian Open gathered pace with a 6-7 (5/7), 6-2, 6-7 (4/7), 7-6 (7/4), 6-2 victory over friend and fellow American JJ Wolf.
Shelton is only the fourth man in the past 20 years to reach the Melbourne quarterfinals on debut.
"I really focused in the fifth set, just being energetic, trusting my fitness and just hustling, being courageous and I thought I did a really good job of competing at the highest level in the fifth set," said the 89th-ranked American.