Raonic, who lost last year's final to Roger Federer, is one of three seeded Canadians, alongside Felix Auger-Aliassime and Denis Shapovalov. Raonic is making his comeback after missing the clay-court season with a right knee injury.
Shapovalov was due to play Germany's Jan-Lennard Struff but their match was put back to Tuesday due to the rain. Earlier, Miomir Kecmanovic rallied to beat German veteran Philipp Kohlschreiber 4-6, 7-5, 6-3. The 19-year-old Serb will next play Shapovalov or Struff.
Among Tuesday's other scheduled matches, it's the fifth-seeded Gael Monfils vs. American Steve Johnson, Auger-Aliassime vs. Ernests Gulbis, and Matteo Berrettini vs. Nick Kyrgios. The Stuttgart Open made the switch from clay to grass in 2015.
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