Fonseca was the talk of Melbourne Park after taking down Russian ninth seed Andrey Rublev in the opening round of the Australian Open, his Grand Slam debut, on Tuesday evening.
Djokovic, who stayed on track for his 11th Melbourne Park title by beating qualifier Jaime Faria on Wednesday, said he had met Fonseca for the first time just after his second-round match.
"I've been following his rise, and I just love how he plays the big points. He's courageous, a very clean hitter, and an all-around player," the 24-times Grand Slam champion said.
"I see some of my game there in his game, particularly when I was his age when you're kind of carelessly going for the shots, just showing what you're capable of.
"He's got the goods definitely to go very far, he showed that last night on a big stage. The future is bright for him, no doubt, if he keeps going this way."
Djokovic said he had been impressed with the way Fonseca had dealt with the pressure of playing a night match on one of Melbourne Park's showcourts on Tuesday.
"It's exciting times for Brazil, but also for the whole tennis world because such a young player and person to be able to play so good on a big stage is impressive," he said.
"It's super important for our sport to have a very good player coming from Brazil. I think they haven't had a player maybe of that calibre since Guga Kuerten."
Djokovic had a close look at two other Grand Slam debutants in his first two matches at his 20th Australian Open with both 21-year-old Faria and American teenager Nishesh Basavareddy taking a set off him.
"I've had plenty of matches in my career ... where I'll play opponents that I've never faced before ... and they just come out firing with basically nothing to lose," he said.
"Overall it was another big test. Another three-hour match, back-to-back against the youngsters. Again, I'm happy with the way I ended up the match. Good fashion, good mindset."
'Incredible' Fonseca
Alcaraz was also suitably impressed.
"What can I say about it? Just incredible. I mean, the way he played his first Grand Slam match against his first top-10 player, it's unbelievable," the Spaniard said after easing into the Melbourne third round.
"The way he approached the match, the way he handled everything, the nerves, the match in general, it's been fantastic.
"He's going to be there," he added of the 112th-ranked Fonseca making it big.
"It's just the beginning of the year, just one win in a Grand Slam, but he's going to be there.
"We're going to put Joao Fonseca's name on the list of the best players in the world really soon."
Fonseca backed up his NextGen crown by winning an ATP Challenger Tour title in Canberra this month.
He then powered through qualifying at Melbourne Park and is now on a 14-match win streak with his next opponent Italian Lorenzo Sonego, who is ranked 55.
"For sure, the NextGen gave me confidence. I mean, I'm playing great tennis, so I need to enjoy this moment, to play my best," Fonseca said after beating Russia's Rublev.
"I'm playing good, so I'm feeling confident.
"I think when I arrived here, my first goal was to qualify for the main draw. Of course, my expectations are bigger now," he added.
"I want more and more."
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