It avenged his loss at the US Open in September to the 20-year-old Russian.
"I'm feeling good physically. The heat didn't scare me at all today, so that's a good sign," said Dimitrov.
The 26-year-old reached the semi-final last year in Melbourne and capped 2017 with victory at the ATP Finals in London in November. That triumph propelled him to a career high ranking of number three in the world.
In the prelude to the Australian Open he reached the semi-final of the Brisbane International tournament where he lost to the Australian Nick Kyrgios.
But his performances at Melbourne Park have not offered up the habitual lessons in stylish shotmaking. He needed five sets in the second round to dispatch the 22-year-old qualifier Mackenzie McDonald. And he was far from sleek in his match against the fast rising Rublev.
Dimitrov served 15 double faults among his 61 unforced errors. Worryingly, 26 glitches came from his supposedly stronger backhand side.
“These are the most important matches for me, when things are not working and I’m able to find a way,” said Dimitrov. “I knew what to expect. I knew what I had to do. Certain things in my game are not working at the moment. I’m just pleased to win the match."