Djokovic braced for 'big battle' with Alcaraz at Australian Open
The 37-year-old Serb, who is gunning for a record 25th Grand Slam title and 11th Melbourne crown, beat 24th seed Jiri Lehecka 6-3, 6-4, 7-6 (7/4) on Rod Laver Arena.
It sent him into the last eight at Melbourne Park for a 15th time, a record he now shares with Roger Federer and one ahead of Rafael Nadal.
His reward is a showdown on Tuesday with third seed Alcaraz, who is already a four-time Slam winner aged 21 but has never gone beyond the Australian Open quarter-finals.
"We had some long battles, long exchanges," said Djokovic, who stopped only very briefly for the post-match on-court interview, his hasty retreat getting some boos from the crowd.
"The kind of matches that I played against him remind me of my match-ups versus Nadal in terms of the intensity and the energy on the court.
"He's a very dynamic, explosive player. Incredibly talented. Charismatic player. Great to watch -- not that great to play against," he added with a smile.
Alcaraz progressed after Jack Draper retired hurt during their last-16 match when the Briton was losing 7-5, 6-1.
Djokovic and Alcaraz have played each other seven times with the Serb leading 4-3, including victory in their last clash in the Paris Olympics final.
They have crossed paths at Grand Slams three times, twice in the Wimbledon decider with the Spaniard winning on both occasions.
But they have never played at Melbourne Park, where Djokovic has achieved his greatest success.
"When we are seeing him playing, he seems like he's young again.... it's unbelievable. He's in a really good shape," said Alcaraz.
But he added: "I'm just ready and I know what I have to do in quarter-finals."
Alcaraz 'feeling great'
Ahead of Melbourne, Lehecka of the Czech Republic won the Brisbane International, where Djokovic lost in the quarter-finals.
But Lehecka was never seriously in the reckoning on the big stage.
Djokovic quickly put pressure on his serve and achieved a break in the eighth game of set one when the Czech sent down a double fault.
Another break on Lehecka's opening serve set the tone for set two with Djokovic dominating from the baseline.
The young Czech changed tactics in a closer set three, pushing Djokovic to the net more while picking up his serving intensity.
It went to a tiebreak where the Serb produced some stunning shots to seal the win.
Against Draper, Alcaraz was well on top when the Briton pulled the pin on a sweltering afternoon because of "multiple areas really in pain".
The 15th seed Draper needed five sets to win his first three Melbourne matches, rallying from behind in all of them to stay in the tournament, 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.
"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."