Carlos Alcaraz is satisfied enough with his Australian Open form to give himself a rating of "seven or eight" out of 10, but he knows he will need to lift his intensity against his next opponent, Miomir Kecmanović.

Alcaraz advanced to the fourth round at Melbourne Park for the first time in his career after Chinese wildcard Shang Juncheng was forced to retire when the world number two led 6-1, 6-1, 1-0 on Rod Laver Arena.

Serbia's Kecmanović moved through to face the reigning Wimbledon champion via a five-set defeat of American 14th seed Tommy Paul, who was a semifinalist last year.

Kecmanović, who is ranked 60 in the world but has been as high as 27, will be the underdog against Alcaraz.

But he has reason to back his chances.

He gave Alcaraz a scare in the 2022 Miami Masters quarterfinals, with the Spaniard prevailing in a third-set tiebreak before going on to win the tournament.

It was a match two-time major winner Alcaraz has not forgotten.

"I remember that match in 2022 in Miami," Alcaraz said at Melbourne Park on Saturday.

"He played an unbelievable game. Myself as well.

"I think it's going to be the same war. He has beaten big guys here in this tournament, so I have to be prepared … hopefully to take it in three sets, but nobody knows.

"I need to play at my best level if I want to move on, so let's see how it's going to be."

Alcaraz has only dropped one set so far during his Australian Open campaign, saying he is "feeling great".

"I think I'm improving every day, every match that I'm playing I'm feeling better and better moving, hitting the ball, and, of course I'm getting used to this court as well," Alcaraz said.

"I think everything is good."