The world number two raced to a 6-0, 6-2 victory over German Altmaier, posting his quickest win of the year in just 58 minutes.
Sinner will face fellow Italian Flavio Cobolli in the last 16 on Thursday as he continues his bid for a fourth title of the season.
The four-time Grand Slam champion had to retire with cramps earlier this month in his third-round match against Tallon Griekspoor in Shanghai.
But he beat Carlos Alcaraz, Novak Djokovic and Stefanos Tsitsipas in an exhibition event in Saudi Arabia last week and was in fine form again in the Austrian capital.
Sinner did not face a single break point in the match and lost just six points in the opening set as he blew the 51st-ranked Altmaier off the court.
"I feel like in the first set everything was working very well," said Sinner.
"I'm very happy about today's performance, starting the tournament in a very positive way, not only game-wise but feeling-wise."
Earlier, Daniil Medvedev continued his strong form by defeating Nuno Borges 6-4, 6-7(7), 6-2.
The former world number one ended a two-year wait for an ATP title last weekend in Almaty.