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.
Former Mexico international Carlos Vela opened the scoring with Raul Ruidiaz firing home a 73rd-minute penalty to settle the All-Stars game.
The 6-2 6-4 second-round loss to Belinda Bencic at the National Bank Open followed her first singles win in more than a year.