The world number one powered through a rapid-fire opening set and overcame a wobble as he started the second set by dropping serve.
The defending champion prevailed in a brief 71 minutes as he claimed an eighth victory in a row at this event and his 30th this season.
"I felt great on the court today, you could see that," said the Italian, who turns 24 on Saturday. "But every day can be different. Let's see what I can do in the semis."
Sinner, playing his first tournament since his Wimbledon triumph, will face either French qualifier Terence Atmane or Danish seventh seed Holger Rune for a place in the final.
Auger-Aliassime meekly lost the opening set after less than 30 minutes, delivering three double-faults in the final game.
After dropping his serve early in the second, Sinner broke back to level at 2-all, eventually advancing as Auger-Aliassime coughed up his eighth double fault on match point.
"I served well today, that was the key for me," Sinner said. "I had a small drop in the second set and I'm happy that I broke back. He moves and serves well, he's difficult to play."
Atmane followed up his victory over fourth-ranked Taylor Fritz with a 6-2, 6-3 win over ninth-ranked Rune.
"I don't think any words can describe how I feel right now," the 136th-ranked Atmane said after blasting 22 winners past Rune, 14 of them off his powerful lefty forehand.
"It's pretty insane to be honest," added Atmane, who will move inside the top 100 in the rankings for the first time.
"I cannot believe it. Being here in the semi-finals of a Masters 1000, breaking into the top 100. It means a lot to me."
In the day's only other men's match, fifth-seeded Ben Shelton - coming off a title in Toronto - posted a 6-4, 6-4 victory over Jiri Lehecka to book a quarter-final clash with third-seeded Alexander Zverev.
Gauff sets up Paolini clash
In women's action, French Open champion Coco Gauff beat Lucia Bronzetti 6-2, 6-4 to reach the quarter-finals.
The second-seeded American's game showed minor rust after a bye in the previous round.
"I could have maybe made some more first serves in that second set, but overall, I had a lot of aces and unreturnables," said Gauff, the 2023 Cincinnati champion who went on to win the US Open title that year.
"I'm happy considering where (my serve) was last week. I'm always trying to get better, but also just trying to take the little wins along the way."
"I'm just trying to be happy with where I'm at now, but still doing the things I need to do to improve."
Gauff, who was broken once in each set, finally secured a spot in the last eight with a concluding break of the 61st-ranked Italian's serve as Bronzetti hit long after 79 minutes on court.
Gauff next takes on seventh seed Jasmine Paolini, who crushed 2024 Wimbledon champion Barbora Krejcikova 6-1, 6-2.
Krejcikova had treatment on her left foot in the second set, and her movement was clearly hampered.
Paolini, the 2024 Roland Garros and Wimbledon finalist, won seven straight games on the way to a 4-0 second-set lead and rallied from 0-30 down in the final game to advance.
"She's a great player and can do anything she wants with the ball," Paolini said. "But I don't think she was at 100% today.
"I'm glad I stayed in the match mentally to the last point."
Gauff will play Paolini - whom she beat here two years ago on the way to the title - in her third 1000-level quarter-final of the season after Madrid and Rome.
France's Varvara Gracheva rallied to beat Germany's Ella Seidel 2-6, 6-1, 6-1 in an error-strewn battle of qualifiers.
"I made her play her best game," Gracheva said. "She was so hot at the end of the first set that I could barely see a ball.
"I had to go back to basics, I had so many mistakes. I was doubting myself but I stuck with it."
She will face Veronika Kudermetova, a 6-4, 6-3 winner over Poland's Magda Linette.