Alcaraz into Australian Open quarters after Draper retires
The Spanish third seed was well on top when the Briton threw in the towel when down 7-5, 6-1 on a sweltering Rod Laver Arena.
The 15th-seeded Draper needed five sets to win his first three Melbourne matches, rallying from behind in all of them to stay in the tournament.
He had spent nearly 13 hours on court to Alcaraz's six and it finally caught up with him.
"It's not the way I wanted to win. But obviously I'm happy to play another quarter-final here in Australia," said Alcaraz, who is bidding to win the title for the first time.
"But a little bit sad, you know, for Jack. He's really a nice person, he doesn't deserve to get injured."
Draper pulled out of the United Cup in the lead-up to Melbourne with a hip injury.
Alcaraz is into the last eight at Melbourne Park for the second successive year.
He has never gone further -- beaten in 2024 by Alexander Zverev -- and will have to beat either Djokovic or Czech Jiri Lehecka, who play later, to do so.
The Australian Open is the only major missing from four-time Grand Slam winner Alcaraz's collection.
Should he go all the way, he will become the youngest man to complete a career Grand Slam -- winning all four majors -- in the Open Era and the youngest to win in Australia since Djokovic in 2008.
"I'm happy with the level I'm playing. On the court and off the court I'm feeling really comfortable here," he said.
"Physically, I'm feeling great. So coming into the second week of a Grand Slam it is important to feel well physically because right now the matches are even tougher.
"But I am ready."
Alcaraz was beaten by Draper when they last met, on the grass of Queen's Club in 2024, but with the hot sun pounding down in Melbourne his booming groundstrokes caused instant problems for the Briton.
The opening set went on serve until the Spaniard seized an opportunity in the sixth game to break on the back of two net winners.
Draper kept plugging away and his persistence paid off with a break to haul back to 5-4 before Alcaraz broke once more to take control again.
The Briton slumped 3-0 behind in the second set and was clearly struggling, finally pulling the pin when he lost the set.