With the victory, the second seeded Swiss, a five-time champion on the Cincinnati hard courts, becomes the first player to reach 300 wins in the ATP Tour's elite Masters 1000 events.
Neither player was able to capitalise on early break chances before Federer eventually won the first set tiebreak on a forehand error by the Canadian.
Pospisil, Wimbledon doubles champion with American Jack Sock, battled back to take the second set but a focussed Federer wasted no time taking charge in the third with an early break putting him on the way to clinching the milestone win.
The day featured a pair of early upsets with Taiwan’s Lu Yen-Hsun shocking fourth seeded Czech Tomas Berdych 3-6 6-3 6-4 while Poland's Jerzy Janowicz knocked off Bulgarian seventh seed Grigor Dimitrov 6-4 3-6 6-3.
In evening matches, Frenchman Gael Monfils sent 13th seed Roberto Bautista Agut home with a dominant 6-4 6-1 win in just under an hour.
The world number 23 gave his 18th ranked opponent a hard-court lesson, winning 80 percent of his first serves on his way to victory, setting up a date with Federer in the third round.
After a first round bye, eighth seeded Briton Andy Murray got his tournament off to a convincing start by easing past Portugal's Joao Sousa 6-3 6-3.
Murray, who has twice hoisted the trophy in Cincinnati, faced just two break points in the entire match, saving both, while disposing of his Portuguese opponent in a tidy 72 minutes.
"The conditions are completely different here to Toronto," said Murray, who made a quarter-final exit in Canada. "A few of the guys that have come in late have struggled with the conditions.
"I hit the ball well from the back of the court today. I was able to control the ball well.
"That was, for me, the most pleasing thing about the match today because I have seen a lot of the players have struggled so far."
Top-ranked American and 11th seed John Isner blasted his way into the third round after firing 16 aces in a 6-3 7-6(1) win over Australian qualifier Marino Matosevic.
Fifth seed Milos Raonic of Canada made light work of American wild card Robby Ginepri 6-2 6-2 but ninth seed Ernests Gulbis saw his tournament end at the hands of American Steve Johnson 6-4 6-4.
Other seeds to survive included Marin Cilic, David Ferrer, Fabio Fognini and Tommy Robredo.
(Reporting by Steve Keating and Ben Everill; Editing by Peter Rutherford)
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