The seeds had varied results on the opening day of the Coupe Rogers, with two of the four who played suffering defeats on Monday.
While Nick Kyrgios and Roberto Bautista Agut comfortably advanced to the Round of 32, John Isner and Lucas Pouille both lost in straight sets.
Kyrgios was in particularly impressive touch, taking just 50 minutes to see off Viktor Troicki 6-1 6-2, but Juan Martin del Potro had the most notable result of the day, defeating the in-form Isner 7-5 7-5.
In other results, Richard Gasquet had a tough time dispatching Canadian wildcard Brayden Schnur 7-6 (7-5) 5-7 6-1, while David Ferrer came from a set down to defeat Kyle Edmund 6-7 (5-7) 6-4 6-3.
Also, Gael Monfils, Benoit Paire and Adrian Mannarino all progressed, with the latter defeating Daniil Medvedev in straight sets 6-3 6-1.
CLINICAL DELPO MAINTAINS OPENING-ROUND STREAK
Juan Martin del Potro provided a huge upset in his win over John Isner, who was undefeated for eight matches coming into Montreal, after titles in Newport and Atlanta.
Although Isner served up a mammoth 24 aces, the Argentine was able to return the ball at crucial periods of the matches.
Failing to capitalise in the ninth game of the second set, del Potro once again broke in the 11th with a booming forehand, in a game that lasted eight minutes.
A 2009 finalist in Montreal, the 35-year-old moved to 11-0 in opening-round matches this season with the victory.
KYRGIOS MAKES LIGHT WORK OF TROICKI
Nick Kyrgios picked up his first win in over two months in a dominant display against Viktor Troicki.
Retiring from his last three opening matches, the 22-year-old Australian showed little signs of the hip and shoulder injuries that have plagued him recently, winning 88 per cent of points on his first serve.
Troicki's serve was faltering in contrast and the 16th seed was brutally effective, winning 70% of points on the Serbian's second serve.
He will next face either Paolo Lorenzi or Frances Tiafoe.
POUILLE STRUGGLES CONTINUE
Lucas Pouille fell in the opening round to a 20-year-old American for the second consecutive week, losing 7-6 (7-5) 7-6 (10-8) to Jared Donaldson.
Unable to recover from last week's defeat to Tommy Paul in Washington, the French 13th seed could not relieve pressure on his serve, having to save seven of eight break points.
Donaldson, who finished with 10 aces, failed to secure the break at 5-4 in the second set but was gifted the match on a double fault in the tie-break.