Pablo Carreno Busta, the 2022 National Bank Open champion, opened the tournament Monday by beating Canadian wild-card entry Liam Draxl 2-6, 6-4, 6-4.
Carreno Busta won his first Masters 1000 title when the tournament was played in Montreal three years ago.
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Another Spaniard, Jaume Munar, advanced with a 6-3, 6-0 victory over Dan Martin, a Canadian who came through qualifying to secure his first ATP Tour main draw appearance.
There were no seeded players in action Monday at the tournament in which top-ranked Jannik Sinner, No. 2 Carlos Alcaraz, No. 5 Jack Draper and No. 6 Novak Djokovic had already withdrawn.
Among the notable results, Australian qualifier Tristan Schoolkate beat rising Brazilian teenage star Joao Fonseca 7-6 (5), 6-4; and American Mackenzie McDonald topped Belgium’s David Goffin 6-4, 6-4.