British number two Dan Evans, meanwhile, shocked eighth-ranked Andrey Rublev.
The British sprinter edged out reigning world 200m champion Dafne Schippers.
Serena Williams is a three-time winner in Montreal but could not muster up another victory fresh from declaring her plans to retire.
Ex-Arsenal forward Carlos Vela and Seattle Sounders striker Raul Ruidiaz scored for the hosts in Minneapolis–Saint Paul.