"I expected a great level from her. That's why I said to myself to stay focused from the beginning to the end and probably that's why I was able to win this one in two sets."
There was little between the pair again in a 54-minute opening set on Thursday with only a pivotal eighth game separating them.
It went to five deuces on Shelby's serve before a rasping crosscourt forehand and a scream of "Come on!" from Sabalenka brought up a third break point.
She converted for a 5-3 lead and served out the set to love.
Sabalenka kept up the pressure on Shelby's serve at the start of the second set, breaking for a 2-1 lead.
The Belarusian was now on a confident roll, peppering Shelby with winners off both wings as she broke twice more before ripping home a backhand crosscourt pass on her first match point.
Sabalenka, who reached the semi-finals at the US Open last year, extended her 2023 unbeaten record to six matches after winning the recent Adelaide International without dropping a set.
She is aiming to get past the last 16 in the Australian Open for the first time and will next play another American Lauren Davis or the 26th seed from Belgium Elise Mertens.
"I've played a lot of matches against Mertens, and I always had battles against Davis," said Sabalenka. "It's going to be tough but I am looking forward to it."