Four-time grand slam singles champion Sinner wasted no time in crushing Galan 6-1 6-1 on Saturday (local time).
World No.1 Sinner opted to take some time off after his Wimbledon win, his first title on the hallowed grass courts in England, where he beat two-time defending champion Carlos Alcaraz.
Sinner was certainly in pristine form at the ATP 1000 hard-court event after a first-round bye, producing just four unforced errors.
Sinner, the defending champion in Cincinnati, next plays Canadian Gabriel Diallo.
"It's very difficult to play here, I feel like the ball is flying, it's very fast," the Italian told Tennis Channel after closing out the match in 59 minutes.
"So when you lose confidence with a couple of shots, it makes it very, very difficult to play."
In other results, Frenchman Arthur Rinderknech upset Norwegian 11th seed Casper Ruud 6-7 (5-7) 6-4 6-2, while eighth-seeded Italian Lorenzo Musetti lost to France's Benjamin Bonzi 5-7 6-4 7-6 (7-4).
Australian interest will be ramped up on Sunday (Monday AEST), with three Aussies to play their second-round matches.
Leading the way will be sixth seed Alex de Minaur, who has been in scintillating form in the lead-up to this month's US Open.
De Minaur recently clinched his 10th career title by winning the Washington Open, which he followed with a quarter-final showing at the Canadian Open in Toronto.
His impressive form has given him an ATP Tour-leading 24 hard-court wins for the season.
De Minaur opens his Cincinnati campaign with literally the tallest order in men's tennis, towering 211cm American Reilly Opelka, who has one of the most blistering serves on tour.
Another Aussie in action will be 21st seed Alexei Popyrin, who also reached the quarter-finals in Canada.
Popyrin takes on Spaniard Martin Landaluce, the former world No.1 junior.
Rounding out the Australian challenge is Adam Walton, who faces the daunting prospect of taking on 12th-seeded Russian Daniil Medvedev, a former world No.1 and the 2021 US Open winner.
The Aussie trio will be out to break a long-standing drought in Cincinnati, where no Australian man has claimed the title since Pat Rafter 27 years ago.
The Cincinnati Open is a key lead-up to the year's final major, the US Open, played on the hard courts at Flushing Meadows in New York from August 24.
- with AAP
