Ronaldo has now scored eight goals in his last five matches.
Andrea Pirlo's side were unable to add to their tally but Ronaldo's brace was enough as they moved into second place, albeit having played a game more than the majority of the teams around them.
Juve dominated from the beginning and Federico Bernardeschi saw his goal disallowed in the 11th minute as Alvaro Morata was offside in the build-up.
He will be especially eager to secure Champions League glory with Juventus - Andrea Pirlo's side are now second in Group G with six points from three games.
"We needed a match like this", said Pirlo. Lazio won 2-0 at Crotone earlier Saturday and Atalanta was surprisingly held 0-0 at Spezia.
Atalanta are sixth, equal on points with seventh-placed Lazio after eight games.
The Old Lady broke the deadlock in the 38th minute as Ronaldo cut inside from the left before firing an unstoppable shot.
Having played an important part in that second goal, Demiral almost added Juve's third as his powerful header cannoned off the crossbar.
The Italy global was also involved in the second goal, sending Joaquin Correa through just before the hour mark as the rain briefly subsided. Crotone remained bottom with just two points from its opening eight matches. But it went ahead despite the ball barely rolling and water splashing up from the waterlogged pitch as the players ran.
It was the fourth Serie A goal in six games this term for the top scorer in Europe last season, who had been out for three weeks because of Covid-19.
Gian Piero Gasperini's Atalanta stalled before their big match at Anfield in four days' time, being held to a second consecutive stalemate.
Diego Farias hit the woodwork for Spezia with Atalanta's Duvan Zapata denied by the post before the break.
The visitors nearly won the match late on but Spezia goalkeeper Ivan Provedel pulled off a stunning double save to deny first Mario Pasalic and then Robin Gosens.