In a tense match on Rod Laver Arena, the two-time Australian Open finalist started off wobbly but turned things around by tiring Ruusuvuori in the early hours to win 3-6, 6-7, 6-4, 7-6, 6-0.
After losing the second set to a one-sided tiebreak, the Russian changed equipment and soon began to dominate, winning the next two sets to level the match then racing past Ruusuvuori in the decider.
He delivered a total of 23 aces, in contrast to his opponent's one. Of those 23, 16 came after the second set.
"Then I (changed my rackets) and finally found one where I felt like I was playing better. Sometimes it is just something you create in your mind," the third seed told reporters.
"I stayed with this racket. I started with it at the beginning of the third set and stayed with it until the end of the match.
"It was not easy to start the match and in the first set I was missing too much. I was missing all over the place."
Medvedev looked at his best in the final portion of the match and said the titanic duel "will remain in his memory".
"If I was a fan, maybe I would have gone home and watched the match on TV. I would have watched 30 minutes and gone to bed. So I thank you from the bottom of my heart for sticking around, you guys are very strong," he added.
Medvedev continues his bid for a second Grand Slam title when he faces 27th seed Felix Auger-Aliassime (23) of Canada in the third round on Saturday.
